It takes two to party. If something is not right in the relationship, then one will not want it. And it doesn't have to be an obvious wrong. 22 years old with 3 kids, one that is almost half her age, she is barely ready for a marriage let alone a ready made family.
And to say they are in love with each when she is not interested in sex doesn't make much sense?! If they are in love then the most intimate of acts will not be a chore or to do item and will be a spontaneous and meaningful act that actually will reduce stress, and make you feel better about yourself and the problems you may have.
In this case, maybe she does not enjoy sex with him because it does not fulfill her needs/desires or what she thinks sex is supposed to be like. Maybe she is not good at it so he just acts mechanical to get it over with. Whatever the problem is they need to figure it out.