the first monstergirl that wish that their husband has a harem very nice from her. but is this really so easy? if two or more monstergirls want sex with the man at the same time problems are not prevent. even when the man can have a harem not anyone wished this when he only wanted sex with the bicorn and not with other monstergirls. love and sex has many forms and even a monstergirl can do things againist their nature.
Well, consider this: in this universe, the current lord of all monsters is a succubus, and it's pretty much all because of her that this situation exists in the first place. Also consider that succubi reproduce by turning other women into succubi. Long story short, creatures that can transform women would probably do so, and whether they would stay afterward is a good question. Not saying it doesn't happen. But if it (lesbian relationships) do happen, it is likely very rare, and is likely to happen mostly with the monsters with a greater degree of self control over their emotions and mind. The dragons come to mind.
As for why not share...alot of these girls are rather...attached to their man. A lamia probably won't share, ever; jealousy is more or less their defining character trait. Others...maybe, but look at their personalities and descriptions. Dark elf is the only one i can think of right off the bat, and of course there is the bicorn here.