Do you want to work on improving your relationship with your husband or are you planning on leaving him still? Your posting history suggests you've more or less already checked out of the relationship/you're about ready to call it quits, but it really doesn't seem like that's what you want if you're struggling with the idea of your spouse and friend going at it in front of you.

If you want to improve your relationship, my opinion is that it is a really bad move to invite a third party (someone you consider a friend, more so) into your bedroom. From what I understand, couples who are able to "make it through to the other side" relatively unscathed are in healthy, strong, stable, and communicative partnerships - things that are missing from your own relationship, it seems. Build up on those aspects first before pulling others in. Try and strike a deal with your husband; get through the pregnancy, work on getting in shape, show him that you care about the health of your relationship (take him* up on the bowling suggestion, even if all you do is sit there, sipping a soda - the point is to spend time with him, doing something outside of the day's usual activities of completing chores, working, taking care of the kids, etc).

Note: I'm not giving him a free pass here, btw. If everything you've been writing this past month is true - I've doubts (1) because it's the internet and (2) because I think it's odd you posted this here instead of places like /r/sex and /r/polyamory (even though I'm pretty sure they'd also caution you not to go through with this) - your husband needs to work on improving himself as well, regardless of if you stay or if you go (some of the things you've mentioned would definitely raise flags for me). I hope you all get some counseling of some sort (couple and/or individual) - you're both dealing with heavy matters and a professional may be more beneficial in digging out the real root of the problem(s).