Some men can't handle love and sex at the same time. So things start off steamy and end up passionless, as the love increases. There is a book I keep plugging called "Mating in captivity" - no I don't have shares in it, but it does explain this one quite well.
Debt will also kill sex, but I have to tell you, unless he works on it, frequency is only going to get worse. Marriage will probably be the final nail in the coffin.
The deprived party in a marriage that becomes sexless usually ends up feeling irrationally un-sexy, and often fails to pluck up courage to leave the person due to feeling that no one else will desire them.
I have been doing a lot of research on mismatched sex drives, and what has emerged so far is that when men are the lagging party, the situation is tricky. This is because be it nurture or nature, men are in general much better able to compartmentalise sex from everything else.
Controversially, I don't believe men have higher sex drives than women. I believe that women simply have a greater capacity to allow censorship or repression of the desire for sex within themselves. If anything, I would say women have the higher sex drive, and capability. especially when you take into account that most men have a refractory period after ejaculation, whereas women generally have no corresponding come-down, or if they do it is certainly not to the same extent.
If he can't shape up, I would get out now. Do not get pregnant what ever you do, otherwise you will probably be stuck with him. There are plenty of women, who would be quite happy with sex once a month. Let him find a match with such a woman. Then there will be no tension.
Actually with two low sex people, there might not be any sexual tension, but there will also probably be no passion or love or much of anything else. I say this because sex is the spark that drives humanity onward. But in our twisted society, it is often viewed as dirty, over-rated, and low down on the list of priorities for a long term happy marriage.
On the site below, I give a method to boost a man's sex drive. It turned me from 3 times a week to unlimited! At one point when we had the time, we were up to 3 times a day. Semen Retention
One question I would ask him: Is he masturbating and ejaculating outside of sex?