An interesting question... and I have a perspective yet described here.
Me? I am a sadist and a sensation Dom. I am married, very happily, to the woman that I feel is my other half. I am in no need of a girlfriend. What I don't have, because my wife is also very dominant, is someone to hurt. Because of this, the vast majority of my play has no sexual element to it, outside the obvious erotic nature of BDSM play anyway.
I don't want the 'Mmmm's, I want the Ouch-Do-That-Again's.
That said, I will most certainly play with a newbie, for a wide variety of reasons: First and foremost has already been mentioned... I am not looking for a relationship, and though the first time is special, it rarely leads to the one you will settle with for the rest of your life. I like being the one to introduce a new sub/bottom to the lifestyle because I have no vested interest in keeping them in that lifestyle... if we play and she decides it's not for her, then I haven't just lost my playmate AND my mate in the same go.
Secondly, Everybody has to be new at something...being new at something is no reason not to try it... I know what I am doing and (as mentioned) I have no ulterior motives, which pardon if this sounds arrogant, makes me the perfect doorway into the lifestyle. Now.. I WILL take far more precautions with a newbie than with someone I've played with many times before, but that doesn't mean I will hold back. As long as she is breathing regularly, can still tell me her name, and wants more, I will give more. I will take her as far as she wants to go, within reason. All that said, there are certain types of play I'd never do with a newbie... like cutting or fire play.
Thirdly... I almost feel it's my responsibility. There are a lot of flakes in the lifestyle... on both sides of the slash. Newbies make such easy prey for experienced predators, so I like to give any newbie I find some perspective. If I can get to them first. then four sessions later the subject will know her (or his) tolerances, will know what areas of play they like and what they don't, and they'll know that they don't have to tie themselves to one person in order to occasionally get spanked.
Now I know many of you ARE looking for the relationship side of things, and on that I'd say: If I had been having sex with a woman for some time... long enough to know her body well, then yes, I'd give her her first scene. On the other hand, if I don't know her at all, then I'd rather it be one or the other at first,.. either BDSM or intercourse, but not both at first.
Finally... even if I did find a subbie that I felt I wanted a relationship with... even if I fell in love with her and wanted her at my feet nonstop... I'd still not offer her a collar for a long... long time. I would never considering collaring a newbie in any way short of a protection collar. I certainly wouldn't want her fostering any ideas of me being her 'One' or her "Only'.