If he's holding out for 15-20 minutes, that's normal and pretty typical. Sex isn't like porn where guys can go the full hour and a half or whatever.
He might be more aroused by a kind of rape fantasy (for lack of a better term) where he turns you down and then ravages you later. He may feel like since you've taken on the role of mother, it's no longer your "place" to proposition him for sex.
Just some ideas. I don't know if they apply but I thought I"d throw them out there.
There really isn't much you can do other than talking to him about it. And it sounds like you've already done that.
But I think it's pretty dumb that he has any kind of say over how you do your make-up/hair or if you masturbate. That is smurfing stupid and controlling. Why would you put up with that?
<blockquote><b>Quoting ~The Lunar Flower~:</b>" SO & I started out in our relationship lusting after eachother so badly that even a small kiss from me ... [snip!] ... hell am I supposed to do if he won't have sex with me as often as I'd like? This whole once every 2 weeks smurf is killing me"</blockquote>
To make things better, I'd back off and go at his pace. Let him initiate.
I think telling him you are going to masterbate will not make things better.
Relationships go thru phases. Just because he is less interested doesn't mean anything is wrong and needs to be fixed. Just be patient and REALLY receptive when HE is ready.