it can easily happen if you're not on Layla's and/or Elena's path.
you can also be on their paths, and screw it up later, allowing an encounter with Sienna.
also, Sienna seems very secure in her relationship. so they could be open to swinging. if Layla and MC are also secure, it could happen.
there are tons of possibilities.
and honestly, this game is all about drama, so "can't imagine how this would end well for the MC" is clearly not a good reason for not doing it... you can literally try to go for Alexis and Layla at the same time. you think that would end well ?

same for Erynn and Alexis... Alexis's secret path is about Erynn pushing her into MC's arms WHILE he is with Layla... on Layla's path, MC fools around with both Layla and Elena... on the very same day, you can fuck Tori, Iris and Layla, and some phone sex with Cara... the only one that seems immune from drama with MC is Valeria for now, she shuts it down really quick.