Venus is love. Mars is sex.
Easy to remember. It also loses about half the meaning.
Venus is connected with values, taste, pleasure, attraction, what we enjoy, and what we consider worthy of our attention. That is why Venus can be visible not only in romance, but even in what a person willingly spends money, time, and energy on.
Mars is connected with action, initiative, drive, the way we pursue what we want, express anger, and protect our interests. Sex fits here too - but it is far from the only expression of Mars.
Relationships simply make both themes especially visible. What attracts me? How do I show desire? Do I make the first move? What happens when we want different things?
Venus does not describe our entire capacity to love, and Mars does not describe our entire sexuality. One planet is simply not enough.
And that is one reason a natal chart is more interesting than astrology memes: a person keeps refusing to fit into one neat formula.
The stars speak. The choice is yours.