These two signs might not click in the sexual department due to the vastly different ways they approach relationships. "Cancer needs a lot of a lot of reassurance and emotional stability, [whereas Leo tends] to be very much in the moment, which is great and spontaneous, but not so good for someone who needs a bit more nurturing on a consistent basis," Monahan says.

These differences can become apparent in bed, too, and they'll quickly realize they don't have much chemistry. "Of course, all relationships can work but you might want to recognize, if you are a Cancer, that you need a less fiery type in your life — someone who will ground you instead (the earth signs for example)," Monahan says. "The trick is to find someone who understands and complements your sign."

Cancer and Leo could work, but they'd have to be wary of toxicity forming between them, if they don't make an effort to understand one another.

Another duo that may want to think twice before jumping into bed is Virgo and Sagittarius, thanks to their positions on the astrological chart.

"The 'square aspect,' which is planets that are separated by 90 degrees, is another difficult aspect that doesn't tend toward sexual chemistry," Monahan says. "Virgo and Sagittarius are an example of this; two planets that could irritate each other and not in that good, sexual tension way."

Instead of playfully bickering as a way of flirting, for example, these two will actually get into arguments. "The perfectionism of Virgo will grate at the happy-go-lucky Sagittarius ways," Monahan says. And if they don't know how to tone that down, they might not have any fun as a result.

"Capricorn and Aquarius are next door neighbors in the heavens and would tend to be bad in between the covers," Monahan says. Again, this position leaves signs feeling disconnected and "alien" to each other, and that dynamic rarely makes for amazing sexual chemistry.

Their personalities can clash, too. "Capricorn is practical, stable, and can be somewhat demanding," Monahan says. "Aquarius is independent, adventurous, and ices out people who make too many demands." And that combo can be a turn-off for both.

A relationship like this can work, but it won't be without its difficulties. These two will want to go into an intimate relationship knowing they won't naturally see eye-to-eye, and look for ways to get to bridge that gap by getting to know each other better.

In terms of their positions on the astrological chart, Leo and Pisces are in what's called a quincunx aspect, which means they are roughly 150 degrees apart.

"Planets involved in a quincunx don't understand each other," Monahan says. "In this instance of Leo and Pisces they are not only of a different element (fire and water) they are also of a different modality (fixed and mutable). It is very difficult to see common ground and hence compatibility between these two signs."

Not to mention, Leo likes to receive lots of attention, Monahan says, and prefers to be at a safe distance from too many emotions, which Pisces has trouble doing. "Leo likes things to be fun, grand, upbeat," she says, "so while Pisces may well be the Zodiac’s romantic, [their] idealistic version of love and sex is very different to Leo's."

It can make for a bumpy road for these zodiac duos, especially if their personalities are mismatched, or they have other astrological goings on that make it difficult for them to connect. But since sexual chemistry is a complex thing, it's something a couple can work on improving as their relationship progresses.