Yes, for most people this would be a deal breaker. Travel planning was challenging if not interesting. But I gave it a shot anyway. When I was in college, my bf at the time, was the same size as me, and we borrowed each others clothes. (though for the record : He borrowed more of my things). I didn't mind.

In the same fashion, suddenly, you have the perks and miles from 2 different airlines & alliances. But then reality prevailed - the bf came running to me when he realized it wasn't so convenient to get from point X to Y on Oneworld. We ended up using Star Alliance. (an option we would not have had if we were both Oneworld, loyal to the same airline). Diversify. @:-)

But I will say this, if you do date someone who is practically married to another airline, be nice and helpful (especially if the other person has no status with your airline and is otherwise without any perks), with things like seat assignments, upgrades, irrops. Your trip (and probably your relationship) will go a lot smoother.