That's what would happen under a proposed statute that's being promoted by Massachusetts state representative Peter Koutoujian, and is being supported by District Attorneys Joseph D. Early Jr. and Gerard Leone.
I suspect that this isn't the goal of the drafters, but that's what the language would call for, when (as I'm pretty sure happens quite often) the cheater has sex afterwards with the regular lover without disclosing the cheating. Here's what the proposed law says:
Whoever has sexual intercourse or unnatural sexual intercourse with a person having obtained that person's consent by the use of fraud, concealment or artifice, and who thereby intentionally deceived such person so that a reasonable person would not have consented but for the deception, shall be punished by imprisonment in the state prison for life or any term of years. As used in this statute, 'fraud' or 'artifice' shall not be construed to mean a promise of future consideration.
The law was apparently proposed in response to some recent fraudulent-sex incidents, one in which a man had sex with his brother's near-sleeping girlfriend pretending to be his brother, and one in which a medical technician conducted an unnecessary pelvic exam (with his fingers, I think) after having pretended to the woman that it was necessary and that he was trained and licensed to perform such exams. But it goes much further than that: Any time someone has consensual sex (1) having gotten the consent through (a) lying or (b) concealment, and (2) a jury (or perhaps a judge) concludes that "a reasonable person would not have consented but for the deception," that's a felony, labeled as a form of rape. Promises ("I'll marry you") are excluded, but other statements — or silences — are not.
So let's see how it plays out in the cheating situation. Alan and Beth are lovers. Beth has sex with Carl. She doesn't tell Alan (or, if Alan confronts her about his suspicions, denies it — that doesn't matter for purposes of the law), but then has sex with Alan again. That, under the law, is rape, so long as the jury or judge concludes that a reasonable person wouldn't have consented to have sex again with his lover had he known that she had cheated on him. Naturally, the same would apply with married couples, but this isn't even just a revival of criminal punishment for adultery — there's no requirement of marriage. (Note of course this would apply regardless of the sex, or sexual orientation, of the partners.)
The same could of course arise in lots of other contexts. A woman conceals from a prospective lover the fact that she'd been a prostitute, or even had had a lot of sexual partners. When they have sex, under the proposed law the man will have been raped as a result — depending, of course, on whether the jury or judge decides that a reasonable person would care about a lover's past prostitution, or even a lover's past promiscuity. (Let's assume that she doesn't have any sexually transmitted disease; there are some narrow laws that mandate revealing STD's to prospective lovers, but those are indeed limited to revealing STD's and preventing the spread of disease. They certainly don't cover all things that a reasonable lover might consider in deciding whether to have sex.)
Likewise if a man (or a woman) gets sex by falsely saying "I love you" (as opposed to "I will always love you" or "I will marry you," which is excluded), again if a jury finds that a reasonable person would have considered this. Same if someone gets sex by lying about his or her wealth or his or her age.
And of course all this would require the case-by-case, jury-by-jury development of the Law of Reasonable Sexual Criteria, as Massachusetts courts have to decide whether a reasonable person would treat a sexual partner's poverty, age, promiscuity, infidelity, and other attributes as sexual deal-killers. (Would it matter, by the way, how appealing the other person otherwise is? Would the jury have to decide whether the "victim" would have had sex with the "rapist" in any event, because the victim was so infatuated, or because the rapist was so hot? "True, Angelina Jolie didn't tell the victim that she was still in a sexual relationship with Billy Bob Thornton, but a reasonable man would have had sex with Angelina Jolie no matter what he knew about her"?)
Just awful. I do think some kinds of sexual frauds could properly be criminalized, for instance if the defendant impersonated some other specific person whom the victim knew, or if the defendant lied about having a serious sexually transmitted disease (or even concealed such a disease), or if the defendant lied about whether certain sexual contact was necessary for medical purposes. But these would be narrow and precisely drafted laws, which would cover a small range of clearly highly reprehensible and unusual conduct, and would not cover behavior that is either proper protection of privacy (e.g., not revealing one's sexual history) or that is an extremely common human failing (e.g., cheating).
UPDATE: Commenter Arkady hints at another hypothetical: Woman fakes orgasms, or more broadly tells the man that he's a great lover. Man says that he wouldn't have continued having sex with her if he'd known that she wasn't having orgasms -- or that he wouldn't have continued having sex with her if he'd known that she was willing to fake something like that, or to lie about his prowess. A prosecutor claims that a reasonable person would indeed have reacted the way this man says he would have reacted. The jury, or the judge deciding on a motion to dismiss, is supported to decide whether a reasonable person (not the average person, mind you, but a reasonable person) would have reacted this way. How on earth is a jury or a judge to make that legal decision?