The two on the bottom left?? They could be, but they could also be a straight couple, or family, or honestly, just friends. That level of intimacy as displayed above is not uncommon at all, especially in Asia.
Knowing the feel of the manga, I highly doubt it would happen like this. If they do get married or something of the sort, it'd probably involve some rather dark and possibly gruesome way of doing so. Not that I'd complain.