She “harbours rot within her” so if you went in then, yeah you would most certainly get scarlet rot
Actual fungus cultures who thrive on the body do so best in the dark damp places. Feet, armpits, crotch. STDs are mainly contracted from the fluids. Fluids contain any viruses, but also act as a “catalyst” for things like genital warts to make more complete contact easier thus making it far more likely to contract.
In the rot’s case it would definitely infect you almost immediately, it’s a mythical fungus that can affect quite literally any living thing, with acute contact killing its host in minutes or even seconds.
The only scarlet rot we see from malenia is in her second phase, and it is on the highest tier of scarlet rot severity in the game (not all scarlet rot ailment is created equal) which makes me assume she would affect you with very severe rot buildup. You may die from the rot before you even busted(not me, I would humiliate myself instantly the second I dipped into that sexy queen) either way though, you’d die in seconds without boluses.
On the bright side, even if you don’t have armor on she can grab you bare handed and throw you in the air without any scarlet rot effect, so you’d likely be safe letting her get handsy. Furthermore, the spore buildup seems to seace once it reaches her face and there are no signs of spore around her mouth sooo, yeah . .