My Boyfriend Won't Have Sex With Me?

My boyfriend and I have been together 3.5 years, and we've had PLENTY of sex throughout them. But about a month ago, he decided he didn't want to have sex anymore because "abstinence is the best birth control." I can't help but think it's something to do with me because we've never had any scares or anything close to one! 
What am I supposed to think and do? He swears it's nothing to do with me, he just doesn't want kids. Sorry babe, but I call bullshit!