I thought it was at least looked down upon in (edit- pre-Christian) Roman society depending on your role in the relationship. In as much as being seen acting as a woman or subservient was a bad thing , being seen as the ‘dominant’ participant not so much!

Edit: I wonder why the downvote for what as far as I know is entirely factual.

https://en.wikipedia.org/wiki/Homosexuality_in_ancient_Rome

Also I think one has to be careful about perhaps using modern concepts of pansexuality/polyamory on ancient cultures especially when it’s based on another culture’s views ( propaganda?) about them and the Romans weren’t exactly unbiased or always worried about being too accurate when writing about other groups. From what I can see ( being no expert) Roman writers seem to have described Celtic women being shared by lots of men , being able to choose their men , and yet also the Celtic men preferring other me? But how reliable those comments are and whether it corresponds precisely to our idea of ‘pansexuality’ etc can we really say?