Who says it's not allowed in MK? Clearly, Sonya and Johnny Cage get together and have Cassie Cage. The thing is, with the way the story telling has been done, it doesn't really strike confidence in people like me for seeing a good romance story told for this series.

It is allowed. Lol just because some people don't like it doesn't mean its not allowed.

I don't think it's unallowed in the strictest sense of the term, it's just that MK is not that sort of series. Having a brutal, horrifying, and occasionally nightmarish story rather than a historical drama-like one, MK doesn't really leave much space for "mushy" elements. At best, you may have, as you pointed out, hints at the relationships (Liu & Kitana, Johnny & Sonya, etc.) but not the detailed depictions of the relationships themselves. I don't find it likely that those who applaud the scenes of head-ripping, skin-flaying, throwing the opponent into acid pool/ fan/ lava, etc... will also be the fans of gentle scenes of emotion exchange. Of course, they may relish watching those things in general but NOT within the same game/ movie.

Personally, if I were to add a touch of sentimentality of my own, I would welcome certain indirect signs of "blasts from the past". E.g. we already have the indication that Cage and Sonya were.... uhm... together... and that they begot Cassie. Perhaps some other original combatants mingled and left offspring, too. I always persistently imagined how would a child of Shang Tsung and Omegis look like, or a child of Shao Kahn and some of his anonymous mistresses from the past, or of Rain and Kitana.... They have done it with Cage and Sonya (may I add, I think I wasn't the only one who never expected their entanglement would actually come to be, especially regarding Sonya), I don't see why the same wouldn't go for another couple.