Incorrect. The punishment for refusing to LAY with her was what you described. In essence, he raped her. The punishment for that is quite a bit more severe...
The closest I know of for a man & wife having intercourse but withdrawing beforehand would be Lev 15:
“If a man lies with a woman and has an emission of semen, both of them shall bathe themselves in water and be unclean until the evening.”
The concern in context was with touching semen, but that is about as specific as scripture gets. There is nothing in Onan’s case that resembles a man using a condom when lying with his wife.
There was no rape. Onan had every right to have sex with Thamar, she “belongeth to him” (Deut. 25:7). The law of the levirate was not to have sex once, but to generally have a marriage. If Onan took Thamar, had sex, but no baby were conceived due to some physiological reason, Onan would have fulfilled the levirate anyway.
The death punishment is for “detestable thing” (Gen 38:10) which is described in the preceding verse: spilling the seed.
God punished by death not sex with a lawful wife, but specifically contracepted sex with her.
The Lev 15 supports the view, that spilling semen is to be avoided while semen that goes where it is supposed to go (and, of course touches the woman) is fulfillment of a Divine command. The difference is that the cleansing is all that is required since there is no intention to spill it; Onan, however, intentionally spilled his semen.
You can argue that the Bible detested contraception for specific socio-economic condition of the time, but as far as the biblical evidence against it, it is plainly there.