This is an idea where someone does something sexually for the benefit of someone else. Usually, within a relationship, it means one half does something sexually for their partner to make the other half happy.
That is, the activity wouldn’t have been their first choice, but they are happy enough to go along with the act.
For instance, a partner has a fantasy that their other half doesn’t necessarily share but goes ahead with taking part to make their partner feel happy.
A partner may not necessarily be into feet fetishes, but plays along for their partner’s benefit, or has a threesome with another person in order to satisfy a fantasy for their partner.
The word Altruism means a “selfless concern for the well-being of others”.
The term was coined by Auguste Comte in about 1850.