the crucial mistake i think people make is looking at farcille as being in an already committed relationship before the story begins. the feelings are there but neither of those girls is equipped to confront those feelings during their dungeon exploring days. falin is too used to people being freaked out by her to imagine anyone would be romantically interested in her unless they spell it out explicitly (this is why she seriously considered toshiro's proposal,according to the adventurer's bible. she wasn't attracted to him but thought he might be the only person who'd ever feel that way about her). and marcille does not do this if she fully acknowledged how much falin has grown up at this point in time she would actually snap for real.