I've never been able to continuously date multiple people at the same time. Sure, I've gone on a string of first dates in the same week, but never has anything expanded from the initial meeting to dating many people at once. It's not that I have anything against it personally — I just like my alone time and only want to give so much time to dating, let alone to several people. So what do you do if the person you're dating is seeing other people while you're very much not?
It's less tricky than you think, but you will want to consider how you approach this, for sure. It's probably something awkward to think about once you've been dating someone for some time — say several weeks into seeing one another. You could very well be pretty unaffected by them dating other people, but it may eat at you the more it goes on (and the more attached you get). That's why I suggest you address it — but of course, it may be your own personal decision to not see other people, and to not be bothered by the fact that the person you're seeing is still dating. Everyone is different. But if it were me? I'd consider the following.