What to do??

If he has been watching (male) gay porn, then I would be very concerned as well (but if it's lesbian porn then not at all). Bisexual guys don't seem to be very faithful. Well, come to think of it, a rather high percentage of hetrosexual guys engage in affairs as well, but bisexual ones do seem to have a different view on the morals of sex with others while married.

It is also a health issue as gay sex of her partner is far more likely to placed her health at risk then heterosexual sex see the CDC website for more details.

If he had any homosexual encounters in his past at the very least he should get a clear bill of goods from his doctor to show her with special regards to HIV.