They did go a bit into detail about this world's view on marriage (although that may only pertain to the elite of society).
iirc, The first wife is the important one that is done for love an holds the highest ranking inthe harem. While the second+ are more for "forming connections/political ties" and/or are a "marriage of convenience" to avoid marrying someone else or to raise their own status. Its why all the girls were trying to become Makoto's second or third wife with their romance letters.
Aye. If we go too much below the elite, a man wouldn't be able to afford more than one wife anyway, nor would the woman gain any status from being #2 or lower. So, if it's just the single husband+wife, they do it not only for love but to support each other and survive the challenges in life. Within the elite, the first wife's position would obviously be of high importance and dedication, so they wouldn't do it haphazardly for love alone, either, if it's a person with any rationality. Makoto is an unbelievable person of unfathomable means, so if a woman can get over his "goblin" looks, then love is basically all they need, since nothing else is of any concern. There will be money, opportunities, healing magic (potions), security, all that jazz if one manages to marry Makoto.
For #2+, love is of little concern, like you said, and I'm sure any woman with firm resolve could stomach spending some nights with him, in exchange for all the boons. They say he looks like a goblin, but that's obviously not literally true. They don't find his face attractive, but he has got no physical abnormalities (apart from the whole speaking thing) and his body is just fine. As far as anyone knows, he's not a brute, either, and is quite reasonable in normal dealings.