I can think of three positions that allow her to have her legs clinched tightly together, and they are some of the nicer positions for everyone involved, actually.
1. Missionary, except with your legs outside of hers
2. Doggy style, again with your legs outside of hers, but then let her lay forward until she is laying flat on the bed and you're laying on her back.
3. Laying on your back with your legs clenched together, and her laying exactly on top of you....it is hard to get much motion--most of it will be you sliding her forward and back by grabbing her ass.
4. I just thought of a forth...spooning! Not only can she clench her legs, she (or you) can also provide clitoral stimulation...I know you said that won't bring her to orgasm on its own, but I am sure it wouldn't hurt matters if everything else was already being taken care of.
(for some reason the guy is lighter colored in this one)
5. Girl sitting on top of guy, guy stabalizing.
6. Side-by-side facing eachother
