I’ve heard, not recommending it, but heard, that a hot bath will temporarily kill sperm. There’s a reason that testicles dangle, getting looser in hotter climes and pulling up close in colder ones; adjustment of the temperature is needed to ensure that the family jewels keep on optimally doing what the good Lord set them in place to do. However I really believe it an abomination to purposely misuse one’s sexual faculties. When the bible says to keep the marriage bed undefiled, this isn’t just a nice sentiment. From this point of view, anything that is meant to pervert the God given purpose of the sex act between two spouses is a bad thing. Not the unforgivable sin, it won’t scuttle a salvation you have accepted, but you don’t keep doing it unrepentantly if you want to be blessed and want to avoid divine chastisement.
I was discussing human sexuality in America and the culture at large, hot baths don’t really figure in to that, and I’m not personally seeking to reduce my sperm count, nor looking for birth control methods.