Jesting aside, I'm sure it is ok. If we're on this forum, we're prepared to hold a dialogue and that must include receiving the occasional PM. Besides, there's no obligation to reply if one's feeling bolshie!
Yes, the system is there so you can contact other users without exchange of e-mail addresses to avoid privacy and abuse of data issues.
If a person doesn't want to receive PMs there is a facility at the foot of the personal info screen in user options to block them. Default is off...ie set to receive.