But when it comes to sex, it is an innate, biological need and the way society is structured - if you are in a relationship, it is considered very bad to seek it elsewhere. So people need to be very very responsive to their partner's sexual needs. It should never be based on whether I feel like having sex or not
first part, yes. last part--no.
Men and Women approach sex very differently. For Men, it is generally how we assume that someone loves us. For women, it is generally a decision made after love and being "up to it" at that moment is actually very important. It isn't just an emotional thing--it's biological.
It takes very little to get a dick hard, as I assume you know. For many women, it takes a lot to get that equipment ready, and no matter how hard you try, if the boss isn't ready to start the clock, no amount of physical activity is going to make that happen.
Simply because it works one way from the male perspective, doesn't make it so from the female perspective. Yes, we should be fulfilling each other's needs and we do expect give back in relationships...that's how relationships work.
Accept, though, that your definition of need and fulfilling of such needs is probably not the same as someone of the opposite gender.
The way you explain it, it just sounds like more soulless data points: x(conversation) + y(flowers) + c(dinner)^cost of wine = sex! It's like you feel cheated out of some non-existent natural order when you don't get the same result every single time you plug in the same functions.