I suspect Tolkien’s primary motivation in Sam marrying Rosie was a literary device to evoke a return to normality and the settled constancy of the shire, rather than explicit signalling that Sam was heterosexual. I can see what you mean though, and actually your point about Bilbo and Frodo sort of gets to the same place (their disordered lives vs the order of marriage) but I think it’s maybe a case of applying a modern interpretation to something that doesn’t need it.

Heterosexual succession (or, rather, the succession driven by the family unit) was (and still is!) a driver of “ordered society” and given both Bilbo and Frodo subverted this because of Tookish events (Bilbo driven by, and Frodo basically suffering the fall out from) Sam marrying Rosie is effectively the natural end point of things. He is able to marry her because of what came before, and their marrying is a signal that those times are over.

I’m going to have to re-read Tolkien now. I haven’t since I was 15, and this thread made me realise I ought to pay it another visit!