Roman sexuality is a problematic topic for many reasons. It is important to always remember these things as you read the following often extremely offensive sources:
a. the Roman concept of sexuality was not ours. Roman society was extremely and openly hierarchical, and wanted sexual relationships that worked very clearly within that hierarchy. In male-female relationships, it was thought that hierarchy was very clear, as obviously the man there held by default the power. (If they thought he did not, or loved his wife too much,, then they were vicious.) In same sex relationships between men the Romans were not so sure, so they were uncomfortable with two men of equal rank (like Mark Antony and Curio) having a relationship, especially when it extended into adulthood. In same sex relationships between women the Romans were completely confused and struggled, to but it nicely, with understanding what could even be going on there.
b. Romans, or at least some elite Romans including many emperors, were obsessed with Roman citizens making more Roman citizens by having babies. Many of the ancient cultures around them were also focused on this. Child mortality was very high, and the Roman army always needed more recruits, as did the state. This meant that there were intense pressures not only to marry but to remarry once you lost your partner for whatever reason. In fact, after Augustus, the Roman state gave a year of mourning after the death of a spouse and if you did not remarry after that you could face sanctions and penalties from the state.
c. Roman concepts of sexuality do not necessarily apply across the Roman empire. The Romans ruled many different peoples across a huge swathe of territory, and not all of those shared the same concepts about sexuality, sex, the body, or even gender.
a. The Romans had a completely binary concept of gender: you were either male or female, and gender was assigned at birth.
b. Those who did not fit within that binary were at risk of being declared an ill omen sent by the gods and put to death at birth.
c. The Romans (or some of them at at rate) also believed, however, that it was possible to spontaneously change your gender.