I've changed my position on this slightly. Although I don't think sex should be an obligation in a committed relationship, I think there is a problem when someone selfishly ignores the needs and desires of their partner, and only fulfills those desires when it benefits them. So, in some sense, yes, there is a responsibility to please your partner through physical intimacy, with the belief that it's a way to create closeness or at least fulfill a mutual desire, to show love or care for the other, and/or to help sustain the relationship. When someone begins to deny their partner that intimacy, yeah, of course, especially if that partner decides it's their right to make those decisions about whether they can and should be intimate for the both of them, yes, it is a problem.