I need advice

Okay, my fiancé and I have a good sex life. We aren't going at it like horny teenagers or Rabbits anymore, we are both in our 20's and we live together now, there's no need for constant sex as if its gonna be a while till we see eachother. But we do still have sex.

So, half the time I initiate by giving him a blowjob that doesn't get complete because we start having sex. He usually finishes in my mouth. I do not spit, figured I'd put that in. And it upsets my stomach, but only after we've had sex when I initiated, not when I give him a blowjob from start to finish.

What I need advice about is why does this happen?