The grammar mistake is that there is none, they just used 'then' as a transition rather than using 'than' which is a comparison. By using 'then' the owner of this marvelous piece of comic means they are going to cuddle, then have sex. Thank you reading this completely useless comment :D
Pfft, people don't let these people tell you it's your brain doing this. In reality; it's actually..... wait for it....... Magic :D you just reached level 2! You can now read drunk words.
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