Are they fully gay, mostly gay, or occasionally straight? If you ask them in dialogue then you'll get a specific. If they aren't Fully Gay, they still have a small chance to be unattracted to a member of the same sex. The factors are: Beauty-40 + 10% of their lust + 10% of the partner's lust + Random 1-30 to determine. If they are mostly gay the target number is merely 10, otherwise it may be 25, 60, or 75 (the farther down the Scale they are).
If they are fully Gay and gaining stress during same-sex, there is likely another factor affecting it. Are the partners related in any way and is the size of the hole greater or smaller than the penetrating object. Vanilla game also has a few factors such as the sex being consensual, certain actions being stressful no matter what without a matching trait, etc.
The more detail you are able to give me about the event the better I can help identify what happened.