Hi! I can provide some semblance of an answer for you. First, about whether rape of a prostitute was a crime. Prostitutes were infames, meaning that they lacked civil rights, just like slaves and actors. So short answer no: they didn't have the right of consent any more than a slave did, so there was no sense that they could be raped. Catharine Edwards, Unspeakable Professions, is good on this for the social side of things. For the legal side, while I don't know the Digest really at all, I DO know Berger's Dictionary of Roman Law! There we learn that "Relations with meretrices [prostitutes] were not punished as stuprum [sex crime]". This means that laws about sex crimes did not apply to prostitutes.

For the second question no, there was not an expectation that women who were raped (so we're talking citizen women here) should commit suicide. If the woman was raped, she was free from the punishments that could attach to adultery, and the punishment just fell on the man (Berger again). Lucretia is an almost mythic exemplum, showing an extreme standard of feminine perfection. We might note that in that story, the men present tell her that she is not guilty and does not deserve punishment, but she kills herself anyway. So even in that story there is no expectation of suicide, at least from the men present.

Buckle up, this is not going to be a happy answer.

I don’t have anything to add to u/boo_cait’s answer about the story of Lucretia, but I think the answer about prostitution must be expanded. First, although, as noted by u/boo_cait, unenslaved prostitutes suffered legal infamy that restricted their access to the courts and protection under the law in a fashion similar to actual slaves, practically, actual slave prostitutes were very common, and slave trafficking seems likely to have supplied a majority of Roman prostitutes throughout the classical period. Possibly a vast majority, although there is debate about that (there were certainly some unenslaved prostitutes), but in any case, there was a basic link, in Roman thought and practice, between slavery and prostitution that should be specifically addressed.

From a modern, correct, perspective, due to this outsize role of slavery in supplying prostitutes, coerced sex – rape – might be said not only to be licit but to be the default mode of prostitution in ancient Rome. Whether or not force was used in the moment, it was in the background. Even in ancient mindsets this could be recognized in special circumstances that prompted slavery and prostitution to be viewed in a different light – such as when considering illegal enslavement and sale into the sex trade. Flip the light-switch of social status that prompted the Romans to see rape as rape, and suddenly they saw it all along the road to prostitution – to the point that the idea that a formally respectable woman could have been illegally enslaved and wound up in a brothel without having been raped at many points along the prostitute-trafficking pipeline was scoffed at as ludicrous nonsense.

(Or, at absolute best, in ancient fantasy-romance literature, portrayed as a fantastical, extraordinary feat. Brothel-peril is a feature of some surviving Roman novels, and possibly a stock plot of the genre, in which it’s an opportunity for the heroine to affirm her superhuman devotion to her chosen man by somehow avoiding what happens to everybody else in such circumstances.)

Quoting McGinn (see sources section):

Rape was a very real prospect for a brothel prostitute […] Her vulnerability is illustrated by the sarcastic comments of the speakers in the rhetorical exercise recorded by the Elder Seneca about the brothel-inmate who kills her rapist. They question the woman’s ability to avoid rape by the pirates who captured and sold her, the pimp who acquired and installed her in a brothel, and the various customers who confronted her, including drunks, gladiators, and hot-blooded young men bearing arms. The visitors she could expect to receive amounted to “a low and hurtful mob” (“sordida iniuriosaque turba”). In other words, rape was the fate of a woman in a brothel: the place raped her, if no man did.

But returning to the formalist legal view of such things, and thus to the ordinary ancient perspective on “legitimately” prostituted prostitutes…

Broadly, as said above, it is quite clear that the laws of sex crimes (most relevantly, stuprum, and also adulteria) specifically define away the possibility of slaves and unenslaved but legally infamous persons being the victims of such sex crimes. What made criminal sex criminal was transgression against the sexual honor that helped define what it meant to be a respectable person. However, in the eyes of Roman law, there was no way to criminally transgress the sexual honor of an infamous person because they had none.

Enslaved people, as well as people subject to legal infamy such as unenslaved prostitutes, had very limited access to courts and the law in general. They had limited ability to bring cases or, under many circumstances, even testify. The exclusion from civil rights affected access to all sorts of justice: not just the specific case of stuprum.

At least one jurist is indulgent of the man in a scenario that’s nonconsensual beyond the level of general background coercion. Ulpian states that there is no liability for a lust-crazed customer who, locked out by a prostitute, breaks down her door to get to her. The legal question involves other opportunists taking advantage of the broken door to rob the prostitute – and that’s the loss for which Ulpian opines there is no action in court – but the original encounter, which involves her trying to lock the man out, doesn’t involve even what agreement an unfree person can give.

However, we can contemplate whether there might be other relevant offenses besides stuprum, and there is some, doubtful, possibility that these alternatives might not be completely denied. Besides law on sex crimes, law on violence could conceivably apply, although there’s a lack of evidence of its being applied in practice and evidence that brothels were viewed as naturally rowdy and violent places, so it’s not clear whether that avenue may also have been closed. Laws on breach of agreement (in case of a transaction gone bad) might have had some application – the law did at least recognize that prostitutes should receive their fees. These avenues might have been theoretically possible but even if so probably would have relied on obtaining the help of a respectable person in handling the case, which adds another layer of dubiousness. We might imagine that a few prostitutes catering to the elite may have had the connections to make something happen, but many more prostitutes occupied a very low place in society and could not likely form such connections.

Lastly, there is the possibility of self-defense in the moment, which might go beyond the locked door and other passive measures to include active violence against the assailant. Even this was not certainly a legally (to say nothing of bodily) safe route – raising the prospect of being charged for wounding or killing an attempted rapist in self-defence – but there are some stories of a prostitute’s (probably a nonslave prostitute’s) use of violence in self‑defence being upheld. How usual this was is not very clear, and there would almost certainly have been greater legal risk in attempting self-defence against someone themselves respectable and powerful than against someone themselves infamous, or just socially marginal.

Overall, the fact that the application of these other laws to prostitutes can only be discussed vaguely is an indication that the Romans largely kept the problems of prostitutes and the formally disgraced out of court and preferred that respectable venues of the law have as little to do with them as possible. With some potential, but probably not very reliable caveats, Roman law shielded the honorable and not the disgraced.

Sources

The best scholarly source here is McGinn, Prostitution, Sex, and the Law in Ancient Rome, which discusses this particular question pp. 326-328. McGinn’s conclusion, that the legal position on the liability for raping a prostitute is not clear, is a main informant for my post.

Other relevant scholarship includes McGinn (again), The Economy of Prostitution in the Roman Empire: Social History and the Brothel, which provides the above quote on p. 89, and Perry, Sexual Damage to Slaves in Roman Law (but this one mainly concerns “sexual damage” to slaves outside the context of prostitution).

Assembling a list of relevant primary sources would take some work. Information is fairly widely scattered, with bits of information in various legal writings and scattered impressionistic or allusive evidence in other literature, such as the aforementioned romance novels.