Las Vegas' motto may be "What happens in Vegas, stays in Vegas," but I would like you to remember this is simply a marketing strategy. Cheating on your significant other in Vegas is not fair to you or your partner. If you're finding yourself cheating every time you travel, why are you even in a relationship?
Monogamy still applies in Vegas. If you get married in Vegas, your marriage is valid in the entire United States. If you have sex in Vegas and get an STI, that STI comes back from Vegas with you!
So, if you cheat on your partner in Vegas and pretend like it doesn't count, you are a trash person. Absolute dumpster fire. Cheating is absolutely unacceptable and allowing your partner to think that you were faithful when you were traveling WHEN YOU WERE NOT is garbage behavior. Cheating causes trust and intimacy issues that can be difficult to resolve, especially depending on how much time passes between the affair and the partner finding out about the affair.
Of course, Las Vegas is the obvious example here. But, this applies to anyone who travels and cheats. Don't be that person! Be loyal to your partner or break the f*ck up with them! Own up to what YOU really want. Nobody is forcing you to be in a relationship, so make sure you're actually happy with who you're dating. If your first instinct when you go to Vegas is to seek out someone else, you might be happier single.
My point here is to be respectful and loyal to your partner, even when you're traveling. If you find that too difficult, then you're with the wrong person! Relationships are supposed to be an enjoyable part of life, so why not date someone you're happy to be with?