So for this you should check your stats, there will be a hidden lewdness meter that is separate from the lust meter. Then while you are fighting an enemy, for example the wharf roach, it will occasionally use a grab attack. This grab attack will increase your lust, and if it is high enough then you will fail to resist it, resulting in a horny scene. If your lewdness is too high, then you will need to visit Rumia and pay to use her stone to reduce your lewdness.
The best strategy to unlock these scenes is to start a combat encounter, kill all but one of the enemies, and then only use shield from then on. This will prevent them from being able to just kill you, which would result in a different scene. If you have enough health not to worry about it, it is usually faster to just skip your turn over and over, but this leaves you vulnerable to attacks.