If you're looking for the generally informal level of Japanese speech, that would involve using the plain forms of verbs (da instead of desu, for example) and probably also avoiding honorifics or more polite pronouns (I'm not sure which pronouns your characters normally use). The most vulgar level of speech involves ending sentences with "te yagaru" or "te yaru".
If you're looking for how to say "to have sex" (or "fuck" which is used in that sentence), then according to the website Jisho.org it looks like the most vulgar term would be "ippatsu yaru", although "hameru" is also translated as "to have sex, to fuck" and is listed as a colloquialism.
I need some help with some Japanese text I need to write for a very strong-lettered part in my fanfic I'm writing right now:
In a part of the story, one of the protagonists, who speaks Japanese at a native level, makes a comment to her friend, an anthro alien who speaks Japanese at some basic level, that it would be useless to know something about a McGuffin that would help her to know if she is fertile or not, if she does not have no one of her same species to have sex with. The original line goes this way:
What little I know about Japanese doesn't help me much to write something that sounds vulgar and aggressive enough, and online translators aren't very helpful either. Does anyone here know enough Japanese to help me with this?
As a special note, the human protagonist speaks Japanese, more accurately Kansai-ben, through standard Kanto-ben could work here as well since she is trying to speak in a way she could be understandable for anyone understanding Japanese. Her alien friend speaks and understands Japanese at basic level, so Japanese Politeness is not applicable in her case, since the alien doesn't care or mind too much about foul language.