I've had to deal with the muscle tension thing, where I just *can't* seem to relax properly.
The only 'cure' I've found is to take things very slowly with a new partner - do you find it gets easier if you've been with the same person for a while and things seem to ease out more?
The only other thing I can suggest is to maybe look into tantric massage techniques, if you've got a partner who'd be willing to try that with you.

You might try a product that's called strangely enough "Smart Balls." It's 2 oblong shaped balls attached to each other with a retrival string. They are put out by a company called Fun Factory. Actually they were recommended by my Dr to help with incontinence because they help to build up the pelvic muscles and give you move control of those muscles so that you can relax easier during sex. The difference between that brand and others is that brand has smaller balls inside the bigger part that move when you do and it stimulates the muscles. They are rather inexpensive at a cost of about $20.00. Check it out, they have helped me alot.

Maybe you would enjoy doing sexual things with your partner that doesn't have to involve the act of sex. You can always do "other" stuff. As far as infertility, you can always adopt, there are so many children that need a home. You need an understanding partner that loves you for you : )