Assuming you are monogamous in official relationships, if you aren't dating anyone, it's perfectly fine to be having sex with more than one person. However, everyone needs to be aware that you may be having sex with more than one person if you do start having sex with more than one person. And, of course, you need to use protection. Sexual safety is paramount.

In your case, though, it's tricky. Are you going to Vegas to get back together with your ex? You say "I don't know what might happen, but I think we'll have sex." That's a pretty good idea of what's going to happen, though, or at least what the intention is. And it sounds like you have already talked about a long distance relationship with the guy from a couple of nights ago.

So basically, it's fine if you are having sex and dating multiple people, but if you are throwing relationships into the mix, it gets a little more tricky. I just advise you to be careful, figure out what you want, don't make any promises you can't keep, and make sure everyone is on the same page.

Oh, and have fun.