Almost two decades ago, my best friend and I both were interested in the same girl. He was the more outgoing type, so I assumed he'd end up with her, but, lo and behold, she was interested in me. It bothered him enormously that we dated.
But... she ended up breaking my heart (in that first girlfriend sort of way that seems like the end of the world until you experience real emotional trauma later in life). He ended up my best friend, and all three of us are still in touch (her, mainly via Xmas cards, and him, often).
A good friend won't go away because you get the girl. And, in the end, getting the girl might not be the best thing anyway. If she dumps you, he won't be upset anymore (if he ever really was), and if you stay together, well, how can he hold finding a wife against you? Odds are it'll end in a bit (most relationships do) and he'll still be your friend.
Assuming, of course, that she's interested in you. 😉