Well... at least begin them to know the characters.You're exgerating imho, knowing the characters a little is useful but you don't have to do the entire game and certainly not ALL the games (who the fuck did ?). I only finished Summer with Mia and I don't think I'll ever have the strength to finish Lust Theory, it's too long and after a lot of time on that game I can say with certainty that it's not interesting to me. I don't think I'll finish Returning to Mia too, honestly, I prefered the first game. I may play one of the kinky inceton games I didn't do in the future, but never Lust Theory at least. So if I don't play Resort of Temptation now, I'll never do, and I have a lot of fun with that game. Very entertaining.
Not mandatory, of course, but I downloaded and played the other games first (in FF when I was getting bored)