hello! great question! I wrote a large portion of my undergraduate Honours thesis on Roman sexuality, so I think I can be of some help.

First, you are totally right about penetration. Roman elite men (viri in Latin) were regarded as socially, culturally, and legally impenetrable. It was illegal to beat a vir the same that you would a slave. in the same vein, it was socially unacceptable for a vir to be penetrated by any man below his station, such as slaves or freedmen. such men were termed pathicus (Latin for bottom) or cinaedus (an overly-feminine man, passive sexual partner) and described as mollitia - soft. now, this was not a damnation of homosexuality but a damnation upon perceived weakness. how could a Roman man rule over these subordinated groups if he submitted to them sexually? a great example comes from Catullus 16. the poet states (translated myself in the soft-core version, go to wikipediea for the full effect):

'I will penetrate and stuff you,

Aurelius the bottom (pathicus) and Furius the girly-man (cinaedus)'

Here, Catullus is using sexual morality not to demonise homosexuality, but to reinforce his own sexual status as a dominate penetrator.

But this was not really your question, so i digress.

female homosexuality is truly fascinating. the latin term is tribas and we find it being used as a derogatory term for a woman who behaves as a man does. Seneca states that this is drinking beer, eating meat, and penetrating sexual partners (male or female). the act of penetrating another was contrary to the female nature and so makes her neither male nor female, but a tribades. Martial refers to several women as tribades, who perform sex acts on women, despite lacking a penis. the poet thinks this totally ridiculous and an unnatural deviation from woman's nature. (a great quote by a historian I admire: these men were often preoccupied with 'trying to imagine what women could ever possibly do with each other.') I am of the opinion that female homosexual relationship did occur in Rome, despite the limited evidence. however, like male homosexuality, its demonisation was not the act of homosexuality itself, but the fact that women were acting against their gender and taking a dominate sexual role.

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now, women having sex with slaves. again, a very big NO. the idea of social class very much comes into play. remember the story of Lucretia? the Republic's founding myth contains the story of a woman raped by Tarquin, who threatened that either she would have sex with him or he would kill her and leave a slave's body in her bed and state that he had killed them both when he caught them in adultery. it also connects to perceived gender norms. one of the best virtues for an elite Roman woman was pudicitia. this is a complex virtue, but can be loosely translated as chastity. this was not only sexual chastity, but modesty of dress, demeanour, and actions. the perception of an elite woman being too free with her male friends could cast aspersions upon the legitimacy of her children. a very serious accusation for her husband. take Messalina, for example. there is absolutely no evidence that she wandered the streets of Rome as a prostitute. but Suetonius creates this anecdote because it is a powerful demonisation of her character as a good Roman woman, wife, and mother. conversely, the ultimate example of female pudicitia is, of course, Livia, wife of Augustus. she was granted the title of Augusta, deified upon her death, and Sueontius writes of her within her home, weaving clothes and overseeing her household.

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Roman sexuality is a fascinating topic and I hope I've granted you a little insight into its complexity. Leading books/scholars on this area are:

Judith Hallet, Roman Sexualities.

Brooten Love between Women: Early Christian Responses to Female Homoeroticism (source quote)

Catherine Edwards The Politics of Immorality in Ancient Rome,

Langlands Sexual Morality in Ancient Rome (A great source for the importance of pudicitia as a virtue)

Richlin. The Garden of Priapus, (great book on sexual violence in Latin poetry)

Skinner,. Sexuality in Greek and Roman Culture

feel free to ask for clarification/further resources!