I understand that if you satisfy the opponent the fight will be over. But when I nut in a monster during fights that doesn't seem to do anything?

Is it for achievement purposes? Or some general fun xD

Topping is being updated with each version makin' it more and more fun but generally yeach it is just that fun. However it is also a tool  to regain some of ur dignity you lost earlier or is a good option to get out of Wereslut fight without bad end if u'r Bitch lvl 3 and have enough strength to trigger her.

Yes you can satisfy the opponent by either letting them have fun with you or having fun with them rather than actually fighting.  Second option is not always possible and amount of fun time needed depends on the enemy. Examples would be Beast Mistress who is satisfied very easily and Wereslut that takes significantly more attention. 

I don't recall any achievements related to topping so don't worry about that. There aren't many visually represented toppings so far most being text only but if you rly want to see some action like that I believe Harpy, Goblin (female I think) and Centaur may have art when topped by u.