If you're talking about gender:
* Masculine is the most common opposite.
If you're talking about qualities:
* Masculine can also be the opposite, as it refers to qualities traditionally associated with men.
* Masculine can also be rough, strong, aggressive, practical, assertive, logical, etc.
* Feminine can also be delicate, soft, gentle, emotional, intuitive, artistic, etc., so the opposite would be the opposite of these qualities.
It's important to remember that these are generalized stereotypes and not all individuals fit into these categories.
It's also worth considering that some people may not identify with either "feminine" or "masculine" at all.