It might help to have a different perspective on it: the alters are not in actuality separate people. They are ONE person. They are dissociated aspects of one human being. What they aren't is different people. Loving an alter is not like 'cheating' on your fiancee. It is loving an aspect of him.
If you are gonna to marry this guy you are gonna to have to love the host an' all, 'cause they are a package deal, hunny bun.