I don't like BL stories, but I wish luck and happiness to any BL couples that are in love with each other and not violating any 3rd party. Actually any couple (BL, GL, or whatever you wish it to be) in love deserve to be happy. (I'm not talking about any kind of love that's forced on someone, that would be harassment!) Love is a beautiful thing, to be able to love, to give, forgive and appreciate. Don't let other people judge you, life is too short, be happy.
Sounds like your friend B is a homophobia. He doesn't have to accept love from A, but could told A nicely to back off ("thank you but No"). But if A keeps on making advances to B even after B told A that he's straight, that would be welcomed harassment from A.