Luffy is supposed to be the pure, "innocent" protagonist. The thought of Luffy actually settling down with a woman is... incomprehensible, to say the least. Luffy is clearly not smart enough to be a pervert; I am not sure if he even knows the mechanisms behind "intercourse."
Again, there is a lot we don't know about Luffy. We only know him as a pirate, as a fighter, and as a leader. Whatever he does in his personal life... Well, Oda doesn't plan on displaying anything of that sort, now does he? I don't know what Boa has done to him in his sleep or while he was unconscious... Well, I don't even want to think about it.
Luffy clearly shows no romantic feelings for any character in the series, so physical intimacy is a no-no. As for the rubber argument, well... As long as Luffy still has his reproduction organs intact, he should be able to reproduce. Luffy clearly shows that he still has fluid within him (i.e. blood, sweat, urine).
Oda doesn't want a romantic manga, nothing of that sort.
As far as the manga is concerned, Luffy's actions are ambigious. Whether he is or he is not a virgin should not even matter. There is a sense of camaderie among his fellow crewmates, a yearn for adventure. It doesn't matter either way. As long as Luffy is shown with the same bold determination that he displayed since the very beginning, he will ascend to greatness.