It seems we've gotten off topic a bit here. Ultimately the question was how is it so often claimed to "being selfish by marring more than one"? I don't see anything that says a husband is wrong in marrying more than one out of physical desire, or that he shouldn't do so if he is in desire of her.
It's only selfish unloving because the woman thinks the marriage should revolve around her. It doesn't. Or at least shouldn't. That's the root problem, not the desire for another wife. Not much more to be said on that besides all I said before.
I think we have gotten off as well in the primary focus of having more than one wife, where does scripture require being able to lead more than one before taking another? This is adding something in that isn't biblical, a wife is her husbands, his helpmeet.
It's just common sense. If you've got 1 horse hitched to a wagon and can't control it, don't hitch another up. If you haven't built the skills to be able to handle two at once, adding another is just asking for someone getting hurt. Which would probably count as unloving. Granted there are some things which can only be learned on the job.