I do think there could be some law or recognition of long term (for lack of a better term, committed friendship). What it’s called in all honestly would be immaterial. For example (to steal an old TV sitcom’s theme), the Golden Girls was a show about women living together who were friends (one I think was a widow, one divorced, etc.). There is no reason there can’t be some way for them to ensure their ability to support each other if they are committed to living together (for medical purposes, etc.). No one would recognizing that imply any type of sexual relationship. Like for example, allowing them to group themselves together under a single insurance plan. They may not be a ‘nuclear family’ but the commitment to each other would represent a family of friends. Real life example:
washingtonpost.com/local/they-met-in-a-1960s-group-house-nearly-50-years-later-theyre-still-roommates/2016/01/29/3ef27e30-a5de-11e5-b53d-972e2751f433_story.html?utm_term=.47f543b1acd8.
There is also the possibility of chaste gay/ssa people living as brothers and sisters together. They may have discerned marriage to an opposite sex spouse is not their vocation but still need some type of family commitment for financial and other support. Again it’s not marriage but committed friendship. Now would some use this type of legislation for a sexual relationship? Probably. But that would be immaterial to the law as it is not a requirement or encouraged (I mean it could be worded so that two widowed siblings living together could qualify). Not to mention civil marriage law allows divorce/remarriage which isn’t Catholic teaching so not even prior to SSM marriage law didn’t reflect Catholic teaching. Today adult siblings are unable to pool together for insurance so it would make that possible which would ease a lot of health related burdens). So end ramble, but in all likelihood, this would never happen. On one side, people would view it as not enough for rights and want only SSM and that is the end goal while the other side would view it as a Trojan horse. SO it would never even make it to more than my abstract thoughts.