Well also At first I didn't understand so I try to do it in his own perspective.
Simon was shock that Conor is already seeing someone so it's his own way to relieve stress by receiving other peoples love in means of using kiev(Conor's cousin) since Simon already know that kiev likes him.
But still for Simon it was purely sex no attachment/feelings.
I hope you understand cause I know that it's kinda disappointing that he do sexual favor but also he justify it and Conor accepted it.
Or just imagine veronica died earlier so that Simon and Conor won't suffer .

I'm still not ok with the fact that he fucked his cousin, I mean I sorta forgave Simon for the other sexual things that he did because Conor did too but so I did...
I don't understand how he got mad at Conor for getting Ronnie pregnant when he literally had a threesome with Conor's cousin and more other people... I'm fine them being together if it makes Conor happy