Bro's gf and sil like to ask my son to kiss them by the mouth. I don like the idea but everyone else seems ok about it. Why cant they give a peck on the cheek or son kisses them by the cheek? What i feel is, can my son happily happily kiss anyone by the mouth just because the person likes him? Eg: stranger by the road?(ok im very extreme, but just wanna state a general example)

I don't even kiss my own son's mouth haha, not something I'm okay with - even my parents are only allowed to kiss his cheeks. I'd tell them nicely that I don't like it because of concerns over health (adults being carrier of a disease and passing to LO, sometimes turns to death), safety (don't want him to be kissing random strangers) and consent (he needs to learn what are "good" and "bad" touches/behaviour, and no SIL or bro's gf are not allowed to). If they don't listen then be firm and limit contact.