sexual enjoyment is supposed to be MUTUAL.
if a person does not enjoy oral sex but you do, this does not make them selfish, this makes you incompatible.
if a person does not enjoy anal sex but you do, this does not make them selfish, this makes you incompatible.
if you think sex should last 3o minutes but they want three hours, this does not make you selfish, this makes you incompatible.
nobody has a duty to perform any sexual act they do not enjoy.
nobody has the right to expect his/her partner to perform sexual acts they do not enjoy.
and anybody who can get his/her jollies from an act, whilst knowing full well that their partner is hating every minute, is one step away from being an abuser.
seems simple, doesn't it?
I'll add...it seems simple, but it actually isn't.
You say "sexual enjoyment is supposed to be MUTUAL." but then say "nobody has a duty to perform any sexual act they do not enjoy." I'll agree that nobody has a duty to do anything, but with sex there is often a give and take.
For example, sometimes one partner wants sex and the other is tired but they put in the effort because they know that another time the situation will be reversed.
Some sex acts are performed more for the enjoyment of the partner, but the act is performed with the understanding that the pleasure will be returned in some other way.
So I will say that it is selfish if you don't do the sex acts that you don't enjoy yet still expect the other person to do everything the way you want even if it's not enjoyable to him/her.
And sometimes it's not so clear cut. Maybe one sexual position is slightly more enjoyable for one partner, but they do it for their partner. Another time, they'll do something the other one wants.