I traced back to the poster you were responding to (@Daneman). While he seemed to imply that it may have been intentional I didn't read that as a given.
Anyhow, even if it's not accidental, I still wouldn't call it necessarily assault. And no, that doesn't mean I automatically condone doing such a thing. What I am saying is that sex is far more complicated than simply placing constraints on what is and what isn't acceptable.
In any sexual relationship, it is basically a requirement that there is a certain amount of exploration, and trying things to see what works and what doesn't. Sure, one has to be careful not to go overboard, but at the same time sometimes pushing the boundaries a bit can lead to mutual benefit. Try to remember that consensual sex is largely about trust, trusting that your partner won't harm you, and allowing him/her to express him/herself sexually during a very intimate act.
Looking at the scenario under discussion, I would argue that dictating to a guy "you are not to cum inside me" is unreasonable to start with (unless it's part of some kinky S&M roleplay - but that's an entirely different scenario). It's something that both partners need to have a say in and a proper compromise met (eg, why is this a requirement? Are there alternatives? How long is this status quo going to be there? What will she do in return for his compliance?). If a guy has decided he's going to cum inside her in spite of her instructing him not to, then clearly they didn't reach a proper agreement, and to play devil's advocate, one could argue that the guy is a victim of sexual abuse if he is expected to go along with her way even though he doesn't want to. This idea that he cums and suddenly it's sexual assault is a bit ridiculous. It might be time to reconsider the relationship, and I could even agree that it's a breach of the trust that is necessary for the continuation of the sexual relationship, but it's on both parties, not just him.
Of course if he's hammering away and she tells him to stop, but he carries on even though she's actively trying to get him off her, then I would agree that it's crossing over into sexual assault territory.