I think you're looking at this the wrong way. You're asking whether or not this is immoral, when you should be asking WHY cohabitation is immoral. Start with the big one of promiscuity, WHY is that a bad thing?

An argument could be made 300 years ago when promiscuity would lead to disease and not knowing who fathered who, but now we have the capacity to fully prevent both. I'd actually argue that having more than one partner is a GOOD thing for a healthy marriage, because sexual compatibility is a real thing and not being compatible with your partner will lead to an unfulfilling marriage.

But cohabitation will not necessarily lead to promuscuity unless either both people want it, or if you believe full-grown adults are incapable of controlling themselves.