Here's the closest one I could find: Specificity of the neuroendocrine response to orgasm during sexual arousal in men. Also, Wikipedia article on oxytocin says that "The relationship between oxytocin and human sexual response is unclear" and cites multiple studies on oxytocin and orgasm, but none of them seem to show any major effect.

So my impression is that oxytocin secretion per se is not heavily affected by orgasm (there is a short-term rise, but that's about it.) However, orgasm significantly affects two other hormones, dopamine and prolactin (also shown in the study I linked above). After an orgasm, dopamine drops and prolactin rises and keeps surging, supposedly for about two weeks (which seems established, but I don't have a source handy.)

Here's a study that shows that prolactin rises after an orgasm in men and women but sex without orgasm doesn't affect prolactin levels: Orgasm-induced prolactin secretion: feedback control of sexual drive?:

This series of studies clearly demonstrated that plasma prolactin (PRL) concentrations are substantially increased for over 1h following orgasm (masturbation and coitus conditions) in both men and women, but unchanged following sexual arousal without orgasm.

My current crude thinking is as follows:

1. Orgasm leads to low dopamine and high prolactin (oxytocin release is negligible).
2. Low dopamine means low motivation (is the Coolidge effect a hard-coded exception?).
3. High prolactin means satiation.
4. When confronting an ugh field, one needs oxytocin and dopamine, but not prolactin.
5. Therefore it's better to avoid the post-orgasmic dopamine and prolactin changes.