You don’t need to be experienced to write about sex. You do need to do your research, and perhaps step back or “fade to black” if you realise you’re out of your depth.
The reality is that not every single author has experienced every single thing. Jackie Collins once commented this: much of the juicy stuff in her books was things that friends had told her, she hadn’t tried it all personally.
So you don’t need to have experienced every single act and position – you can kind of extrapolate – but authors with zero actual knowledge of the basics frequently make glaring errors.
One issue I’ve seen is authors writing about adult protagonists (ie not teenagers, but independent adults in their twenties and older) taking part in endless, endless kissing scenes. Unless there’s a religious element or something, actual adults progress from kissing. They don’t just neck for hours, day after day, week after week, with no contact below the neck.
Things swell, hands move, urges grow… Men don’t constantly focus on a woman’s lips. Their thoughts and imaginings soon go elsewhere. (Hint: downwards…)
Conversely, the average woman doesn’t instantly “soak her panties” the second she meets the Hot Hero Guy for the first time. Her “vagina” is not “dripping wet”. (Unless she has a bladder disorder!) I also see this a lot in romance stories, and it’s partly unrealistic and partly downright unpleasant to read. Without wanting to sound overly delicate and repressed, it’s wise to avoid being too crude with more visceral bodily details in Romance. Very few readers are turned on by it (unless you’re writing actual erotica) and you’re more likely to turn them off and lose them.
Let your characters feel “heat” or “tingles” but be wary of too much moisture… at least until they’re actually both in bed together.
Ultimately it’s better to simply skip the intimate scenes if you’re ignorant or nervous about that stuff. As a reader I’m perfectly happy to read a novel that “fades to black” for the intimate bits. Occasionally it’s disappointing, but reading boring/bad/implausible sex is way worse.