Hi Researching and welcome !
First, I agree with the overall sentiment of most of the respondents so far that opening a marriage, considering poly etc should not be looked at as a solution for your current problem.
Next, as others have mentioned, it's very common, bordering on 'normal' (whatever that is ?) for new mothers to lose some to all of their sex drive. The focus (mentally, emotionally) is on the child now. It's largely a biological thing.
BUT (big but here) you have at least identified this as a problem - and it IS ! A potentially serious one ! Sex is an important part of being human and maybe more-so for men at some points. Sexual difficulties are probably the single largest reason problems start to develop in relationships. There's lots of others but this seems the most common starting point. From there all manner of things that can follow but often it started here.
But it's not all about HIS needs either ! It's about yours too. You are a sexual creature and sexuality is important to your own well being. It's a great mood stabilizer, good for your heart, good for your self esteem and just plain good in general !
Pregnancy, childbirth, all that goes with that, brings hormonal shock but a lot of it is between the ears. It's natural for this little one to seem to demand all the priorities but I think that you should realize that having a loving, close family (or lack of) will present much more danger to him/her long term !
I highly recommend getting back on the sexual wagon as soon as possible. Do whatever works for you to get there. Get back to masturbating as much as possible and let all the erotic/kinky images play through your head. If you like things like porn, make time to watch together. Think about fantasies you may have had and talk about them. Work towards fulfilling them. In short, try to put some focus on sexual thoughts. It won't be long and your body should follow naturally. Think about it as just one of the numerous things that's really important to building the relationship you want with your SO .
Good for you, good for him, and good for everyone else your lives touch !
You can do it