I'm so sorry to hear that your husband is wanting a divorce. That is tragic.
Absolutely don't get them tied at this point, but before I address why, here's some thoughts. It probably is a good idea to do it due to your high risk pregnancies, and one should not feel they have to "provide the goods" that a potential husband might desire, especially since its based on what you think a man would desire. A real man would be thrilled that you were putting your health (and the health of the unborn child) first, rather then risking it to create a child. You need to make a decision based on what's good for you period. You may never remarry, you might find someone that has kids already, or someone who doesn't want more and loves yours as his own, you just don't know what the future holds. Your reasons for getting one will probably not change too much in the future. Many women are able to find a 2nd spouse without providing them with children.
Now, all that said, here's why I'd say don't do it now. This is a very fragile time for you, dealing with not only a pregnancy but your husband leaving you. Your emotions are all over the place, so its a bad time to make a big decision, like weather or not to tie your tubes. Give it some time, wait until the baby is at least 6 months old and your hormones are more normal, see what your situation will be at that time, and what you think is best for you in the long run, and then make a decision.