If you are single and you want to date and have sex with several guys, it's fine IMHO to go ahead and do that. You should always use protection from STDs.

I am from the school of thought that you don't need to tell one about the other when you are single. If they are smart, they should realize it is a possibility.

If they start to talk about being exclusive, then you need to commit to them and stop seeing other guys.

There's no need to lie. I am not a cheat. If there is no ring on your finger or a spoken understanding of commitment, you don't owe anything to anyone except safe sex.