Is it ok for a single man to give a valentine's present to a married woman?

It is entirely situational. It depends on who the woman is, what your relationship with her husband is, and what your intentions are. Do you feel the situation is appropriate yourself?

If the woman is close friend, like a sister, and you are in a close and trusting relationship with her husband, it wouldn't hurt to give her a little teddy bear or something. If you are truly unsure, ask her husband if he would feel okay with you giving her a little something.

Even if it is a coworker or something, a small non-romantic gift would probably be okay, especially if you got a little something for your other coworkers as well. In a case like that, it becomes more like a nice gesture.

However, if you are wanting to give her something to express you have any sort of romantic interest, then no. In that case, you are violating her and her husband's relationship. You are putting her in an awkward situation, and you might be creating tension that could have been avoided.