Reston, both prior replies make good sense to me (including it could be a self esteem issue related to being overweight).
I am not going to pass any judgment because: [1] I can understand how you are so burdened with all you have going on that you don't feel that you have the time, energy or INCLINATION for sex and [2] I understand how you husband wants and needs sex and will have to get it somewhere else if he can not get it from you.
As DB Lady says, you do have a responsibility to allow your husband to have sex with you (or you should allow him to have that satisfaction elsewhere and NOT complain about it). His normal sexual needs do need to be met.
So, while I understand BOTH sides, I also see a marriage headed for sure destruction (or two people living miserably married and with a lot of RESENTMENT that will build and build and build).
You need to see a marriage and sex counselor about this. You probably do need to try to lose weight (however you did not say your husband objects or complains about your weight and he is STILL sexually interested in you). But more than the weight, you need a counselor who can help you see why and how to balance your life better, to rediscover some sexual pleasure in marriage ... or at least to allow your husband to ... and to save what is a marriage that is going to be destroyed if you stay on the present course ... denying him sex and/or not understanding his need/desire for it.
If you have children, or if you love your husband, or if he is a good provider and is generally good to you ... you need to realize you are about to damage all of that and it might be irreversible (the damage that might be done). At least, get the counseling and have your eyes open and be informed and sure of the consequences that you are putting into motion. This does NOT mean your problems do not need attention and should not also be addressed; it just means you are on a sure collision course right now.
Good luck in school and with the children ... two of the most worthwhile endeavors in life. Don't sacrifice either of those; just make room for your husbands need too (or allow him to have his needs met in soem other manner).
Wish you, him and the family the best,
ROB