sex vs gender

The other night I was out with 2 co-workers and they were asking about how the pregnancy was going. I mentioned we had hoped to have an ultrasound "to find out the gender of the baby" then quickly corrected myself to say "the sex of the baby." They both looked at me, sort of dumbfounded that I would replace "gender" with "sex" saying "they mean the same thing."

I am not well versed on the subject, but to me, the word "sex" refers to genitalia; and as I understand it "gender" is a social construct that polarizes "male" (or masculine) and "female" (or feminine).

Now, I'm not trying to get too semantically deep.  A lot of time I want to be a manly man that is masculine. But a lot of times I feel insecure or whatever because I'm not so much. I'm sensitive, and I cry, and I can sew, and don't know half the rules of football, and hate baseball, and so on and so forth...

Anyways, for our baby I'm aiming to stress gender-neutrality to a certain degree. I don't have a problem with having a boy that gets obsessed with football or a girl that likes pink, or vice versa.

I've not done much to research any prevailing thought or philosophies or insights or the like, but feel pretty confident that what I think makes sense...

Thoughts?