It sounds like the reaction is only when seminal fluid is released inside your vagina.That suggests an allergic type reaction to the proteins in the seminal fluid.
Pertinent questions are: Do you have this reaction if you have intercourse but a condom is used ? Do you experience orgasm with sexual intercourse ? Do you experience orgasm through other types of sexual stimulation and, if so, does the nausea reaction occur ? Have you noticed if the reaction occurs at any particular time of your menstrual cycle?
The main complaint I have heard related to orgasm has been pelvic pain following an orgasm. That can vary with the menstrual cycle and can be due to spasm of the muscles of the uterus.
If the nausea is more generalized i.e occurs with intercourse whether or not the partner has an orgasm, then I would advise a pelvic examination as an ovarian cyst or endometriosis could be triggering the nausea.