Yevamot 34B

והא תמר בביאה ראשונה איעברא! אמר ליה: תמר באצבע מעכה. דאמר רבי יצחק: כל מועכות של בית רבי ⁠— תמר שמן, ולמה נקרא שמן תמר ⁠— על שם תמר שמעכה באצבעה. והא הוו ער ואונן! ער ואונן שמשו שלא כדרכן.

But didn’t Tamar become pregnant from the first act of sex, despite the fact that she was a virgin at the time of her sexual act with Judah? Rav Naḥman said: Tamar broke her hymen with her finger prior to sex, and it is due to this that she became pregnant from the first act of sex, as R' Yitzḥak said: All of those women from the household of R' Yehuda HaNasi who break their hymens are named Tamar by nickname. And why are they named Tamar? They are called this on account of Tamar, who broke her hymen with her finger. The Talmud wonders about the proof from Tamar itself: But weren’t there Er and Onan, her previous husbands, who presumably had sex with her? The Talmud responds: Er and Onan had sex in an atypical manner, i.e., anal sex, and therefore she was still a virgin.

מיתיבי: כל עשרים וארבעה חדש דש מבפנים וזורה מבחוץ, דברי רבי אליעזר. אמרו לו: הללו אינו אלא כמעשה ער ואונן!

The Talmud raises an objection from a baraita: After a woman gives birth, her husband penetrates inside and spills his semen outside for the entire 24 months during which the baby is breastfeeding, so that his wife not become pregnant, as that would terminate her milk production and the child might die. This is the statement of R' Eliezer. They said to him: These acts are nothing other than acts similar to those of Er and Onan, which are prohibited. Regardless, it can be deduced from here that Er and Onan engaged in normative sex with Tamar, only they did not fully complete the sexual act.