This question got me curious: what does it mean to be sexual active but not violate the anti-sodomy laws on the books? The following answer is not from a deep font of knowledge, but is merely what I have gathered through my own curiosity this evening.

Throughout the 19th century there were three acts that outlawed "buggery" and/or "sodomy": the Buggery Act of 1533, which was replaced by the Offenses Against the Person Act of 1828, which was replaced by the Offences against the Person Act of 1861.

None of them were particularly clear with what they were outlawing. The Offenses Against the Person Act of 1828 is the pithiest, so I'll quote the relevant section in full here:

Sodomy.

XV. And be it enacted, That every Person convicted of the abominable Crime of Buggery, committed either with Mankind or with any Animal, shall suffer Death as a Felon.

As you can see, the act does not define buggery or sodomy (neither of the other acts do, either), and all later works I've found seems to assume buggery and sodomy are to be treated as synonyms. In common law systems, the courts set binding precedents for what the law actually means in ambiguous cases.

In 1817, the case Rex v. Jacobs set out to exactly what sorts of "unnatural acts" are included under buggery/sodomy. I found the full decision quoted in a Montana Supreme Court decision, State v. Dietz, and I'll warn you, it's disturbing and the victim is a seven year old child. Here's the decision:

Rex v. Samuel Jacobs.

The prisoner forced open a child's mouth and put in his private parts, and proceeded to a completion of his lust. Held, that this did not constitute the offence of sodomy.

The prisoner was tried and convicted before the Lord Chief Baron Richards, at the Warwick Lent assizes, in the [*332] year 1817, upon an indictment for sodomy, *556 committed on James Thompson (a boy of about seven years of age,) on the 8th of March, 1817.

It was proved very satisfactorily, that the prisoner had prevailed upon the child to go with him from the market place in Nun-Eaton to a rick yard in a field near the town; that he forced the boy's mouth open with his fingers, and put his private parts into the boy's mouth, and emitted in his mouth.

The question was whether this was sodomy.

In Easter term, 1817, the judges met, and were of opinion that this did not constitute the offence of sodomy, and directed a pardon to be applied for.

What does this mean? That in the initial ruling, oral rape of a child was ruled sodomy in 1817 but this initial ruling was overruled upon appeal. The Dietz decision uses the term "per annum" a lot, and it seems from that understanding that in English Common Law in the 19th century, buggery and sodom both meant anal sex not oral sex. As some 19th century legal dictionaries (quoted in the Dietz opinion),

At common law, sodomy, was committed only by penetration per annum; penetration per os did not constitute the crime.

And

Sodomy consists of sexual connection with any brute animal, or in sexual connection, per anum, by a man, with any man or woman. The act committed in a child's mouth is not enough.

Some are slightly more demure and do not say where sodomy is committed, but where it isn't:

To constitute this offense, the act must be in that part where sodomy is usually committed. The act in a child's mouth does not constitute the offense.

The Dietz opinion quotes a lot of these legal dictionaries (before moving onto medical dictionaries, which hold the same), all based on the Rex v. Jacobs opinion. You can read the full opinion here.

It's worth noting that this seems to be a reversal of the 18th-century standards, where oral sex does appear to be included in some sodomy convictions.

As a side note, U.S. law in the 19th century apparently largely followed the 1817 precedent, which is why I was able to find the full text of the option in Dietz. For example, when the Portland Vice Scandal of 1913 happened, the state of Oregon responded by clarifying the law and specifically including fellatio (per os) in the new law. Some other states also changed their laws regarding sodomy or created new laws (contributing to the delinquency of a minor, committing lewd and lascivious acts, etc.) which also covered oral sex.

In the U.K., this sort of amendment, broadening criminal homosexual acts beyond "buggery", happened slightly earlier, in 1885, when Labouchere Amendment to Criminal Law Amendment Act of 1885 was passed. That section reads:

Any male person who, in public or private, commits, or is a party to the commission of, or procures, or attempts to procure the commission by any male person of, any act of gross indecency with an other male person, shall be guilty of a misdemeanour, and being convicted thereof, shall be liable at the discretion of the Court to be imprisoned for any term not exceeding two years, with or without hard labour.

Now, again, "gross indecency" is no more defined in statute than "buggery" or "sodomy" had been (definition would likely have been too yucky), but it was clearly meant to cover homosexual relations that fell short of legal sodomy per anum. This is ultimately the Act that Alan Turing, Oscar Wilde, and the men prosecuted from the Cleveland Street Raid were charged under.

Now, I only looked into this because I was surprised that homosexual oral sex wasn't covered by early 19th century anti-sodomy laws, but I can't find anything that indicates to me that oral sex, mutual masturbation, and other similar homosexual acts were explicitly illegal in England between 1817 and 1885. However, it is clear that they would put one in a marginalized, scandalizing community, and I think Wilde's case (among others) shows that to be convicted of such an offense (and probably even just accused) was to suffer tremendous social opprobrium not entirely tied up with mere matters of the law. Though his trial was after the 1885 making gross indecency illegal, and not requiring buggery, it gives I think a sense of the social climate that would have existed even before the change.

As you'll see below, gay men were frequent victims of blackmail, especially after 1885.

(Continued below with a discussion of the records we have from the Old Bailey Court in London)

In response to the secondary question:

Search of the proceedings of the Old Bailey 1801-1850 shows 140 indictments for criminal sodomy during the period, with 76 convictions.

Prior to 1834 all found guilty were sentenced to death by hanging. After 1848 none were (despite it being a capital crime until 1861).

The last death sentence executed was in 1835. Thereafter, mostly half-year, a year, two-year sentences.

in what situations were people actually prosecuted for sodomy? It seems like a difficult crime to get convictions for.

The last men executed, James Pratt and John Smith (1835) were literally convicted due to "nosy neighbors looking through a keyhole." There's really no alternative to eyewitness testimony--only whether observers caught the accused in flagrante or were themselves complicit.

So we need to separate this question. It's not so difficult to convict (after all, +50% guilty for all indictments) but difficult to indict.

Those 140 case over 50 years often (and naturally) involve two defendants. It's more like two cases prosecuted per annum. Population increase in the jurisdiction is +100% in the timeframe. The population profile tends young, and (of course) all potential criminals are male. Figure the yearly indictment rate is, very broadly, on the order of one-in-a-quarter-million potential sodomites.

So, yeah, difficult to indict because the only evidence is a witness. Discussion of societal norms of who would testify against a defendant, given the potential death sentence involved in doing so (and how this changed given the reduction in potential sentences) is beyond what I'd dare here.

FUN FACT: Thought this is outside the era you define, in the (in)famous Boulton & Parks case (1870) the police medical examiner decided to examine the arrested for evidence of sodomy, and reported that he found such evidence. But his self-appointed proctological investigations proved a disaster for the prosecution.

I'm not aware of any CSI-type evidence being presented before that date.