I was seriously considering the idea of marrying my boyfriend. However, he has just admitted to me he is bisexual. Is there any medication which would make him straight? Or could I get him a course of psychotherapy which would change his outlook?

I'm afraid there is no medication which will alter a person's sexual orientation. Psychotherapy isn't often successful either. Furthermore, very few bisexual people are willing to undergo it since they are mostly pretty happy as they are. Getting married won't change your guy's inclinations so maybe you should consider looking for someone else.