We don't know if Stephen demands sex from the girls for the tuition. Let's assume that Stephen really fucked 3 female students and that they got the school money from Stephen, then Stephen is a lousy businessman. However, for that kind of money I would fuck the three coeds 3 times every week.
I also wonder why Stephen would go after Isabella when he has 3 horny female students he could fuck.
I think it's more so that Stephen is accused of it and he follows these rumors.
Who wouldnt wanna fuck Bella, independent on if your getting some or not ? Brad Pitt was fucking Nicole Aniston on the regular, ring on finger and all, and still stuck his dick into Angelina.
And what do you think Mona ment when she came out of that room crying "I can't do it, not with an old guy!" if she wasnt supposed to fuck him ? Beat him at scrabble ?
And that fact that he admitted 3 to his wife, doesnt mean it was actually 3, bro rule suggests it was more like 6. He most likely fucked Quinn, Riona, Sarah, Melanie and Camila ( who sure as hell knew what Mona was doing when she was waiting for her at the bar ).
And of course he is playing with fire, I said that an year ago. He must a) believe in the old mantra that mutually beneficial deals have a higher chance of staying a secret and b) if it doesnt, he can buy himself out of any trouble or he has the neccessary connections. Still makes him an idiot tho, he can have 18-19-20 year old escorts 2 towns over with nobody being the wiser. They'd be cheaper too.
However I dont think he fucks them every week, its probably once per semester or payment cycle. He wont just shell out the entire money for 4 years for 1 fuck, thats for damn sure.