I'd base a rationale of marriage on the idea of fostering at least minimal companionship which everyone deserves and which thereby deserves protections and special considerations so that someone doesn't have to go through life alone. We do enough of that on our own.
I've zero issues with whatever configuration of relationship whoever finds enjoyable given adult status, consent and competence, but marriage to me seems an institution of two if only for the logistics. If three then four then five then six then crazy.
Also, single people get fucked enough by costs of living, by government & corporate benefits (isn't a pension that extends to a widow worth more than a single guy's pension?), etc., even by salaries (how many managers will pay more a guy who's supporting kids than a single gay guy without kids though doing the same work) so I see no reason to encourage, foster nor certainly to support (will all 20 spouses be eligible for SS benefits of the breadwinner?) multi-marriage which could increase that imbalance.
That three guys want to raise a kid together, assuming they are all loving people I'm sure that could be a wonderful family. Because I didn't have enough issues by the guilt trips of my two parents. What's one more?