It's not a bug, having in mind that you are also able to tease straight males in battle, like the male bandit for a example, so the same can happen to you even if your character is set to homossexual. I think the only thing that it impacts is story elements and enemies male/female ratio, not sure though
Nonsense is right, but maybe as a little counter measure being homosexual could give resistance to you when being teased by the opposite gender, on either male or female MC. Then to balance it a NPC's sexuality could give them resistance to you or add in more points for them. like the bandit twins would be less affected by a man since they complain about you being one if you raise their libido and lose to them, however if they are Bi or Versatile (and both too) would just not gain anything from this and would be neutral to everything. like against the saytr he doesn't mind being on bottom or top so he could be neutral! or depending on which route you have chosen for any character could make any future fights with them become their weaknesses. Like tank could be weak to you if you're a power bottom and he's a service top!
It is also worth noting that if you are aggressive towards the nymph, she will respond in kind. So basically, uh, the more often you stab her to death the less often she'll attempt things (and will note things like 'Ugh, you, you're no fun').
Otherwise, Nymphs in particular are 'literally magic' more than anything else, and the curse of lust is clearly effecting the herbalist just like it is to everyone else around. Any revulsion you feel is similar to how I presume most NPCs feel when you use a love potion or lust potions on them if they weren't otherwise interested, haha.