What does that remind you of?
the only justification for having sex with his brother’s wife WAS to have children.
The punishment for not fulfilling the levirical marriage was that the widow would publicly spit on the unwilling husband wile tasking his shoe (Deut 25:9). but Onan was punished by death, not for what he failed to do, but "because he did a detestable thing" (Gen 38:10). The only thing he did different from any other married couple was, he spilled his seed. That is what was "detestable" and the reason God killed him, Bible tells us.
The question rests on if a man can use his brain to modify the results of what he does
Generally, yes, provided that which he does is righteous act. Using medical technology for the purpose of healing is good as any healing is good. Using the same for a sinful purpose is not good. So the question is not at all whether to use modern technology for achieving contraception but whether to use contraception.
My wife will not get pregnant if if I make love to her tonight, so it is no longer love-making, but something else?
If it is not contracepted sex it is love making, and a righteous act because that is the act that makes babies, and if it is, then it is not lovemaking, because it is an act that only simulates making babies but in fact ensures none is made, -- a lie ad a sin. Her physiological condition has nothing to do with it. I did not make that clear?
“The only thing he did different from any other married couple was, he spilled his seed. “
Incorrect. He was having sex with his brother’s wife, allowed ONLY for the purpose of giving her a child, AND spilling his seed. I think there is a difference between a married couple choosing not to have kids right now, and someone having sex with their brother’s wife under a false pretense.
Couples can seek intimacy with each other without wanting to have kids at that time. I fail to see how mutually agreeing not to have kids equals lying to each other about your love.