Because you can't give of yourself unless you first possess yourself.

I think we as humans are ultimately built for love. Perfect love gives generously and receives graciously. However, you can't give what you don't have, and if you are slave to your passions, you can't fully possess yourself.

In short, sexual continence actually allows us to love more richly.

All that said, it's good you recognize the reasons for your atheism. I think a lot of people reject religion because they like sex, but hide the fact from themselves behind a bunch of other arguments and justifications.