Is dating multiple guys ethical? At the same time? Just kidding. In the last year I was on a few of the dating sites, for the first time. I think it is kind of expected on these sites that you will be checking out more than one person. I did, but I was honest as to my intentions. Most dates were one and done. I think it is also expected on these sites that, if you go out with someone a few times - and especially if sex is involved - that you will disable your account. Be careful with this, because if you are intimate with someone who you do not want a relationship with, they will almost certainly check out your account to see if you are still on looking. If this happens, expect some nasty emails/texts/phone calls. That's not an ethical issue unless you are misleading someone, but it is an annoying feature of these sites.
I learned the hard way to avoid giving my phone number until after I had met them. Email will work just fine, and those sites typically have their own internal email system. If you use yours, make sure it doesn't ID you. I have more crazy ass women stories than I care to relate. Harm no one. Be honest. Have fun. Wear protection (well, make him).
Back to dating multiple people: A few years ago, when I wasn't in the relationship mood (incapable actually) I dated a lot of different women at the same time. Juggling them gave me a charge. It was fun. And expensive. But I never lied to any of them or promised any of them anything, so I didn't have to look over my shoulder or avoid phone calls, etc. It was what it was. I never played on their affections or told them I loved them, they were my soul mate (Tiger Woods comes to mind), etc. If you go that route, just be honest. Promise nothing. Avoid relying on them or planning your day/week/month/year around them. And don't tell them you love them just to get them naked. Oh, that's us guys. Sorry.