User blog comment:DeathGodPrayer/Talk About What You Want!/@comment-24098614-20140131080416/@comment-16281343-20140131081141

That is just a running joke in the series. Kiba doesn't have any romantic feelings towards Issei and there will never be. He just views Issei as his best friend and he feels that he is indebted to him since he had a major role in resolving his past.

~Like what I added in Kiba's trivia section....
 * According to Issei, Yuuto is surprisingly a lecher himself.

If Issei says he is one, then I'm pretty sure he's a straight guy that likes women.