Pisces is the one that needs and seeks a soulmate. Pisces wants to be in a deep emotional bond with its partner, therefore breaking up with a Pisces can be both difficult and easy depending on, it's particularly how to tell the Pisces in words, not real words, but in your actions that you are no longer committed and bonded. That you no longer feel that you are soulmates. If you withdraw, in any way, affection and withdraw from having that what I would call sexual bonding.
It's not necessarily withdraw from sex, but from sexual bonding. If you are someone who goes flitting off at night or you are surrounded with other people and parties and going places and you completely ignore, in a way, the Pisceans emotional needs, the Piscean will slowly, slowly, this may take some time because Pisceans tend to be in for the long haul, but slowly, slowly, the Piscean will start to fall away and withdraw and feel even emotionally wilted.
In many ways, a relationship with the Piscean is like growing a garden. You keep it nurtured, lots of water, which is emotion, lots of feeling, lots of protection that you give to the Pisces. Once you withdraw that, much like a flower, the Piscean will just wilt away and leave.