He is your fiance.. so you are assuming that you will marry him and be together in 10 years time when he is 60, and 20 years time when he is 70. I'm no expert, but men's desire to have sex does decrease with age, plus as you have said, he was never really that into it in the first place. You are 34, at your sexual peak. Mens sexual peak is 18! It sounds like you are just not in the same place, sexually, due to the age gap, and a bit of personality and preference. You both need to compromise to make this work. He is 50 and is going to start winding down a bit from now on. You need to be aware of this! However he does also need to accept that you are in your prime, and need sex more often.
Why not try and do things together- romantic things, that may or may not lead to sex. He could be feeling pressured by you, which in itself is a turn off. If he is exhausted from work, make time to be together. Turn off the TV and PC. Go out for meals and to the cinema/theatre. Dress up! Go away for the weekend to a posh hotel, or abroad. Make more time to appreciate each other, but allow sex to happen naturally, rather than everything being a precursor to it.
If he really can only cope with sex once a month and isn't willing to compromise, maybe you need to accept that although you love him, he isn't necessarily the right man for you to get married to?