>>126227 Among GURPS' vast array of traits for customizing characters, there are a decently sized subset that relate to sex, some generically and some specific.
On the generic side, one may have exceptionally handsome or ugly appearance, charisma, etc. These may be specified to be gender-oriented in nature, or not, as the case may be.
More specifically, one may have Sex Appeal, be Lecherous, or in appropriate settings be a species or altered person who is a hermaphromorph, or sexless. One may take a vow of chastity, officially classified as a minor disadvantage (yes, seriously), or be sterile, which is usually worth no disadvantage. GURPS has a canonical Erotic Art skill, with official bonuses to skill if a character posesses the advantages of Flexibility and/or Double-Jointed. Professional Skill (Courtesan) is found in GURPS Fantasy, though this covers the pre- and post-schtupping ends of things mostly. (It also mentions that professionals with Erotic Art "may prefer spontaneity to technique in their personal relationships," and politely leaves specific effects up to the GM.)
Specific skills may apply to individual encounters, such as Wrestling for trying to bed Kagura, or Knot-Tying for Tomo, or Riding for Rachel Handelbarz.
Finally, be careful who you use all these traits with. Dependents (children) and Social Disease are possible Disadvantages as well. :^)