According to Daizenshuu 7, Goku has knowledge about "se*".

[Eu sou Perfeito]

Spoiler:

Has Son Goku really acquired more intellectual maturity since his marriage?

Goku marries Chi-Chi without knowing exactly what it is. One might have wondered if he would eventually evolve after going through such a responsible stage. And that is the case! Goku becomes less and less ignorant about things in life. Furthermore, he learns to flatter people (like Master Kaio) and becomes comfortable in social relationships. With his last encounters, he also starts talking about sex and lies. If you think about it, it could be that Chichi's influence had a lot to do with her husband's social evolution. And the latter had to subject him to spartan training so that he would become someone people could talk to, which wasn't always easy given how ignorant he was...

[AliTheZombie13]

Inb4 Super!Goku, "Hey Trunks, you put your mouth in hers, why did you do that for?!"

[JulieYBM]

God, I hope he does.

[MasenkoHA]

He has two fuck trophies. Of course he does.

Also given Gohan was likely born exactly a year after the 23rd Tenkaichi Budokai that would mean one of 3 things

1. Chi Chi had a long ass year long pregnancy

2. Goku and Chi Chi had a sexless marriage for about the first 3 months

3. They went at it for a bit before conceiving Gohan

And I lean towards option 3.

[Zephyr]

Also given Gohan was likely born exactly a year after the 23rd Tenkaichi Budokai that would mean one of 3 things

1. Chi Chi had a long ass year long pregnancy

Maybe this is the source of Gohan's incredible power.

[Thanos]

I never gave too much thought to his comments in Super about kissing. It was just a quick gag, not a big deal. I suppose it's possible they consummated their relationship (at least twice...?) without snogging. There are crazier things that have happened in this series, to be fair.

Maybe they could somehow work in that Chichi has a phobia for or doesn't like kissing for some reason, and thus Goku wouldn't have any reason to necessarily know anything about it?

[miguelnuva1]

Goku was talking about Trunks mama birding Mai, not the kiss. Vegeta took the bait and Goku decided to mess with him.

[Saiya6Cit]

I thought it was understood chichi taught him how to do it.

Chichi was a princess. Ox satan was the king of a mountain. Super natural powers run in the family. Therefore chichi was danded to find the strongest man on earth to procreate with him. Let's not forget she did consider yamcha for a moment.

I always admired Chichi's strenght in terms of patience. The patience she had when teaching goku.

Chichi was the best wife. She did housechores, she cooked and educated the children. She also used to make their clothes.
The only one that would have ever worked for Goku.

By the time goku "made" goten he was probably thinking: " this made chichi so happy last time... let me trying as a super saiyan". A bit experimentation I guess...

[Hellspawn28]

Goku had to read Roshi's magazines as a kid. I think he has a clear idea on what sex was.