There's a lot of weird stuff going on here. First of all, someone new joined your lab one month ago and in that month, your friend not only started a personal relationship but even plan to marry? That seems like a red flag to begin with (although to be fair, I know multiple people who got engaged after 2-3 dates and have had long and happy marriages... so not saying that it's a problem, just a red flag).

Second, your co-founder wants to grant co-founder status to his girlfriend. That seems like an odd request. I think I'd be uncomfortable with this even if it was my best friend and he'd been married to his wife for over a decade. There's plenty of things that can go wrong in a startup to begin with, no need to add the possibility of divorce and other relationship issues to the mix.

Finally, they "often reach consensus without involving me in the preliminary discussions"? That's very odd, assuming that you and your friend have roughly equal shares of equity. I personally would shy away from partnering with two romantically involved people, but at the very least, I'd want to feel like I'm not being crowded out and having decisions made behind my back.