Sana did not call homosexuality a mental illness
What she said was "ok then lets look at the theory of evolution"and then she listed the three pillars that has been the fundaments of the theory of evolution for the longest time
1 it's either genetic
2 or a mutation/illness
3 or a choice
She just shoved his ignorance back in his face using his "belief" against him
Clearly he's not really familiar with the basics of the theory of evolution or else he should have seen that coming.
Now this is a spoiler but they will touch upon the subject again. The conversation doesnt end here.