But, I also don't want to sneak around behind their backs like I'm doing something wrong.
Assuming you're not bringing home a new guy every night, you're definitely not doing something wrong, but somehow you don't seem to really believe that yourself, because you also say
...and in later years will sleep around thinking "it was ok for Mom".
Notice how you use "I have returned to the dating scene" and "my girls will sleep around". One is neutral, the other has a strongly negative conotation. Why is that?
Why wouldn't it be okay for your daughters when it is okay for you? Is it just an age thing? Thinking about that, does that really make sense? I'm assuming that with "later years", you're talking about maybe the later teenage years. This is when most teenagers become interested in actually trying sex. For boys it seems to be okay to go through a string of girl friends. Why shouldn't that be okay for girls?
Personally, I think that it helps for kids to see stable, longer-lasting relationships, so they can learn by example. But I don't think that's absolutely necessary; a lot also depends on the individual character of a person, and just because mom used to have a new boyfriend every five months doesn't mean that your children can't get a marriage that lasts a lifetime. If kids just imitated their parents dating behavior, then no divorce kid would every marry, and no kid growing up in a stable marriage would ever get anything else than a stable marriage herself.
The important thing seems to me to make sure your daughters don't run stupid risks when they're teenagers. That's when we're on our most risky behavior because our brains are being rewired, we don't have the experience yet to abstain from the really dangerous stuff, and we want to try out new things. You bringing guys home (and I'm assuming you're not bringing home a new guy every night here) might be a chance to discuss how you decide on who to bring home, and who not to get involved with. I think it's important that you're honest there, and that you can serve as a good role model in that respect - so don't bring home men whose character you question just because they're good-looking, and talking about character, it's difficult to judge someone's character if you just spend a single night with him for carnal pleasure, so I'd counsel against an abundance of one-night-stands. If you're having short relationships, make sure that your daughters get to know the kind of man your BF is, and talk about why a relationship breaks apart - without going too much into the really private parts. That's something your daughters might want to know, and something they'll eventually be able to relate to very well. You might also find a natural way to weave contraception and health issues into a discussion when the time comes for it to matter.
I 'm worried that the girls, seeing them there for breakfast the next day will be confused
I wouldn't worry about that. At that age, it should be enough to tell them the evening before they go to bed that you might have a friend stay for the night. That might already be enough information for them. If you are good at choosing your men, they'll be nice to your daughters, and that will probably be enough for them.
If they wonder where he'll sleep, you can tell them he'll sleep in your room, and leave it at that.
When they get a bit older, they might wonder why boy and girl sleep in the same room, or even the same bed, and when they're a bit older again, they might realize what you're doing at night, and that might be the time to start explaining a bit more, but I doubt that it will be harmful to them to know their mom has a sex life. Usually, it's not something kids want linger on; they'll just accept it as a fact.
Just make sure your dates are good men that treat you the way you'd want your daughter's BFs to treat them.