I'm a 21-year-old female who has an IUD. I've had sex with quite a few men and one thing seems to be almost constant among them: trying to fuck without condoms. Many of the men I've been with seem to be perfectly fine and terribly eager to have sex without condoms. This has always angered me. They generally assume or make sure I'm on birth control, which they immediately take to mean condom-free sex is welcome. I don't want to have sex without condoms without being in a committed relationship. I know people cheat and monogamy doesn't mean STIs won't happen but it's the risk I'm comfortable with. I'm so annoyed now with how often men try to get out of using condoms (it's often persistent and even with people I've been seeing a while) that I kind of want to start lying and say I'm not on birth control. The risk of a baby seems to be the only STI most men are concerned with. Is it alright for me to lie and say I'm not on any birth control and explain why I lied later on if things get serious?

I'm Understandably Distressed

Let's get this out of the way first: You're right, IUD, STIs happen to people in monogamous relationships. People cheat, people lie, people contract, people transmit.

This 2015 study found that people in consensually nonmonogamous (CNM) relationships were no more likely to contract an STI than people in monogamous relationships. The reason? People in CNM relationships were likelier to test regularly and likelier to use condoms not just with their secondary partners and/or pieces on the side, but when necessary use them with their primary partners as well. (If someone in a CNM relationship has an accident with a secondary partner—if a condom slips off, if a condom breaks, if a condom somehow doesn't wind up on a dick for some other reason—the CNMer can ask their primary to use condoms while waiting to get STI test results back. Instead of drawing attention to an infidelity, like someone suddenly asking to use condoms in a closed relationship, the CNMer who asks to start using condoms is drawing attention to their fidelity.)

Moving on...

Right again, IUD: babies do seem to be the only sexually-transmitted infection some men are worried about. Australian researchers conducted a large study about stealthing—the deeply shitty, rape-adjacent practice of surreptitiously removing the condom during intercourse—and they were shocked to discover how common this deeply shitty practice seems to be. Justin Lehmiller (who conducted the STI study I cited above) took a look at the stealthing study:

The researchers estimated in advance that approximately 2% of the sample would report having been stealthed; however, that’s not what they found. In fact, 32% of the women and 19% of the men surveyed reported having experienced stealthing... A majority of both groups reported discussing the event with their partner afterwards, and most also reported feeling emotionally stressed about it. A majority also considered stealthing to be a form of sexual assault. These results suggest that stealthing is not a rare occurrence and we would do well to study it further.

The study did not include heterosexual men. As Lehmiller points out, straight men can also be stealthed; there are some scattered/apocryphal reports out there about women poking holes in condoms before sex or retrieving them after sex. These women, who don't exist in great number, aren't motivated by selfish desire for increased pleasure; they just wanna get pregnant.

We could study the rare (I presume) straight male victims of stealthing, of course, but I would be far more interested in a study examining heterosexual men's motives for stealthing. One question to put to these assholes: Are they more likely to "go stealth," i.e. to sexually assault a woman, if they know her to be on some other form of birth control? Or are they just so wrapped up—or so unwrapped up—in their own pleasure that they don't give a shit about babies or any of the other STIs?

Moving on to your actual question...

Can you lie? Of course you can. Should you lie? In the case of a casual sex partner who might not have your best interests at heart, i.e. some total rando, I think you can lie and should lie. The lie doesn't do the dude any harm; it's not like you're telling him you're on birth control when you're not. And if telling this lie inspires some rando to be more careful about keeping the condom on (condoms can fall off by accident), then it's a lie taht made the sex safer for you and him both. The rando isn't victimized, IUD, the rando benefits.

And if you wind up getting serious about someone—if some dude manages to make the transition from hot rando to hot boyfriend—and he reacts badly to "having been lied to" when you tell him about your IUD, IUD, then you can and probably should say this to him: "Look, I could've waited to fuck you until I was really sure you were a good guy that I could trust. But then you would've missed out on all the awesome sex we've had already. Would that have been better? And can you see the compliment here? I'm basically saying I think you're a good guy, a guy I can really trust, and that's why I'm telling you about my IUD. Turns out, you're not an asshole and I didn't need to lie to you. I know that now, dude, but I didn't always know that because I'm not psychic. Now do you rawdog me or do you wanna complain?"