My significant other views it almost as cheating.

Main Part

BUT, if you (the one with DID) are fine with that stuff happening, probably he won't think of it as cheating (and I mean really OK, not just the "I will let him do anything with them so he will be happy" kind of OK). If the alter wants to, is mature enough to decide, and you aren't hurt by it happening, he has no reason to feel guilty or afraid of hhurting you. Guilt and fear are probably what makes him uncomfortable with that, and makes him think of it that way.
Just think about it, if you are fine with him dating your other alters, he should be fine as well. It will probably help them (and you, too), as it's a way in which your alters can feel supported and loved, and stop feeling like a "sex-machine", as one of them does, or like "tools" for other people to use.
Tell him that it's not cheating if you know about it, and agree with it. Just tell him not to keep it as a secret from you, and talk with your alters about it as well. He should always know when he's dating, kissing or having sex with an alter. That way, he will be more confident, and the fear of hurting you by dating you, you and other you's (xD) will go away.
In the cases when the alter needs to be shown that there's more to a healthy relationship than just sex, it will be a lot easier for him to do that if he is free to date, kiss, hug and show love to her, as he does with you.

I hope some of what I said was clear enough to help you In conclusion, if both of you are open about it, it can't be considered as "cheating", so everything should be OK. Just talk with him, with your alters, and if you can, let him talk to your alters as well, if you consider it convenient.