I'm thinking that relationships come in all forms. Some marriages are open, so I'm not judgemental about the adultery aspect of this situation.

Having expressed that, I go on to wonder why the only solution is that she move in with you and marry you.

Is that just for convenience to eliminate the long commute? Suppose you were next door neighbors and you could get together in under 30 seconds. Would that be enough? If not, why not?