My perspective may be somewhat different than many. I guess we'll see
But I picked up a couple clues from your post that I think are important.
To ME, 'marriage' is largely (if not completely) a legal contract, and should be treated as such. The reasons you mention (primarily) hint at the fact you recognize this also. Your need of insurance coverage (given your condition) and considering marriage as one possible way to obtain this realistically is not unethical in my mind. If your SO and you are completely on the same page regarding this then it makes sense to me. But I would also suggest you take it one step further and put together a good pre-nup agreement you both are comfortable with as part of this. It's well worth the minimal time & effort required. And it can give you both a stronger sense of comfort knowing that you have ironed out some of the potential dirty laundry in advance.
As to the other love (?) interest. I think you used a key word (multiple times).
The term 'boy'. And it sounds like he is exactly that.
My thought is that you can't be risking your health & happiness by trying to have your cake and eat it too - especially if the cake isn't 'done' yet.
You may have to decide to toss that back in the oven and see about testing it later. Not smart to suffer malnutrition - or worse - waiting for the cake to rise to the need.
And if you need more sustenance I'm sure there's a numbers of 'bakeries' not far away where the cakes are ready.
Make sense ?