It depends on the person, I would think.
I think the important part is to make sure you be responsible about it. The primary function of the act is reproduction, so I think anyone going into it, even with precautions, should understand and accept that possibility.
Is it just pleasure or just a relationship thing is simply up to the two involved in at that instance.
If you treat it as just pleasure without a relationship factor, you essentially are just using the other person as a masturbatory aid. Maybe you want to make sure they get their rocks off too, but your endgame is your pleasure. If that's how someone wants to see it, and both parties have that understanding, then fine. If you treat it as a mutual gain, working towards each other's pleasure and such, then cool. If you treat it as an expression of emotion and relationship, then again, make sure both parties understand that. You don't want a pleasure person mixed in with an interpersonal person. So as far as long-term and monogamy issues go, if a person treats it as an interpersonal interaction, they probably don't want to share, but maybe they do with an understanding that with them it is interpersonal and with others it is just pleasure.
For me and my wife, it is an interpersonal thing, and a pleasure thing, also a reproductive thing, and some or all at once. Sometimes we have sex to express our love and affirm each other, sometimes we have sex just to get our rocks off, and sometimes we have sex to have babies. None of those are mutually exclusive in any given instance. We are monogamous because we consider it a part of our relationship, not to be shared with others.
Other people might not consider sex an exclusive part of their relationship, and that's fine, so long as both parties understand and accept that.
So, it comes down to the individuals in each instance. You can't just comprehensively compartmentalize something like sex. Well, you can, but that doesn't mean it will work out.