You can see in the context of the verse, he gave her no children ON PURPOSE because under the law at the time, those children would have had his brother's name and continued his brother's family line instead of letting it die out. And Onan did not want to raise that kid, and maybe coveted his brother's property (which would likely be given to his children if his brother never had an heir).

This verse has nothing to do with "sex for pleasure" vs "sex for reproduction".

It is no longer considered a brother's "duty" to sleep with his deceased brother's widow if they had no children. The entire custom the morality of this scenario hinges on is not part of Western culture, at all. So even if this passage had a moral lesson applied to us today, which it doesnt, it would be about family legacy and inheritance, not sex for pleasure vs sex for reproduction.

It is entirely disingenuous to just put in bold the part where he didnt give her children, and act like all ejaculation without children is sinful.

It's like taking the verse where Jesus tells the fig tree "may no one ever eat fruit from you again" and using it to claim Christians are not allowed to sell groceries