Some of the responses in here
Just because someone is bi doesn't mean their "opportunities" are bigger. Let's not pretend all gay and straight people are the epitome of faithful here, btw. I'm bi and I've been in multiple, faithful longterm relationships, and they didn't end due to "wishing" my partner had different body parts.
If y'all really worry about this, you need to iron out some insecurities before getting into a serious relationship.