No he was just a client
I agree with Kasumi having slept with other men before, whether it was because that is his preference, or because of 'work', or both. And as for his position preference, it doesn't really matter? I really love reading seki couples, so I don't care, the narrative of the relationship is my priority. But since in the story it isn't addressed, feel free to make up whatever you like about his past, because in the end, he's with Yajima and it all becomes moot.
Like Amorim said, the 'sensei'/older lawyer was a client, http://www.mangago.zone/read-manga/kare_no_shousou_to_koi_ni_tsuite/sa/manga/kare_no_shousou_to_koi_ni_tsuite/c004/pg-10/
This conversation means that he used to do 'that' sort of work (seducing people for money, or compromise people for money). Whether Kasumi did that sort of work for the sensei before, or sensei knew him through an acquaintance, which I find more likely, is up for interpretation, so believe whatever works for you in that case too.
his family knows he's gay, so i'm pretty sure he's slept with men before. & the old lawyer isn't important in the story so i don't think he did anything with him
i want to ask about kasumi, so i think yajima is not his first men, and basiclly he is gay, and may be a uke, or fine with both way, i dont know but when i give this manga a deep reading ( yes some time i dont have any thing to do XD) i end up with a logt of question :(
and another thing, did he had a relation with this old lawyer, i mean the ex boy friend of yajima??