It is said explicitly that he is bisexual, not gay, as Isabel would not have married him otherwise in the second case. The marriage was not a pretense, according to her, but became strained following the lost of the child (provided my memory works, which is guarantee of nothing).
About the chances it depends.. these are not independent events, so it is hard to tell, but the point is, that I don't see how he will be more angry about MC having a relationship with one of the girls over his wife. You could argue that he will be angry if MC is doing both his sisters, but again, would that be worst than a guy doing your wife? seriously?