I don't like that idea, no.
Are they both adults? Yes
Do I consider them mature? in some aspects, physically, mentally, yes.
emotionally? No, They've almost never experienced relationships of that sort, their dynamic is wholly different to ours when it comes to interpersonal relationships.
Spartan IIs (and to some extent IIIs) are not the type of people to pair off.
I feel like a relationship would delegitimize the narrative work that's being put into the struggle with humanity John (and Spartan IIs in general, in some cases) experience.
To be honest, I think the type of fondness they share for one another is a lot deeper than one as sensual or sexual as the one you describe, and it's more polygamous that that, too. The love they share is between each other, as a team, and as Spartans. They were raised with the mentality that family is everything, live by the pack, die by the pack. I don't think they would play favorites. Even with human psychology being what it is, That conditioning nut is a tough one to break, even disregarding all the hormonal changes they've gone through with their augmentation. and all of that isn't going to (or shouldn't) be trashed because people like to read between the lines for stuff that probably isn't even there.
Looking at Spartan IIs through a well adjusted human's lens isn't the way to go about it, not entirely, anyway. Especially when it comes to emotional maturity and the dynamics of social interaction.