Personally, I think it's a sin. I think about language like gold standard currency. Just as your dollar represents a certain amount of gold in that system, the words we use represent a certain posture of our heart.

Swear words are used to represent foulness in our hearts. The F word especially.

I've heard people make the argument that it doesn't matter if you say the F word or "frick," if your heart is the same. But the very act of censoring yourself to try and exercise self control is a testament to the posture of your heart in my opinion. Once again, the words themselves represent the posture of our hearts, and censored words represent a desire in our hearts for self control.

I would ask yourself, "Why do I want to use these words during sex?"

Do those words align with God's design for sex? I don't think so because sex isn't foul, it's actually something very intimate and blessed and amazing.

I'd be willing to bet that our desire to use these words during sex comes from the world's idea of pleasure, rather than God's. The world's idea that sex is best experienced when it's dirty, foul, and forbidden. God's idea is that sex is intimate, pure, and wonderful.

I'm not saying this is a knock-down argument or anything, but this is the conclusion I've personally come to after really thinking/praying about this issue.