Whilst it was seen as demeaning/taboo to be the recipient of homosexual sex, there were certain people for whom they either had no choice, or it wouldn't affect their social standing too much anyway. Slaves, prostitutes, entertainers or youths younger than 20 or so (though it's worth noting that free-born Roman citizens were off limits).

So whilst a free Roman man having a male partner wasn't that uncommon, there was in fact a limited pool to pick from, but with the prevelance of slavery/forced prostitution of slaves it wasn't that hard to find a partner. In fact there was a type of slave called a 'concubinus' that some Roman men had before they were married as someone to have sex with (with the slave being penetrated).

So as a TL;DR, it wasnt that common, there were a lot of slaves who could be used (often in what we would call rape today, but since they were considered property at the time, Romans saw it differently), in addition to prostitues or entertainers.