Does a husband sin if he stops having sexual relations with his wife because of her weight gain and snoring, which made him lose desire for her?

If "cohabitation" refers to sleeping in the same bed, it is supererogatory (recommended) and not obligatory, and it is permissible to forgo it for a valid excuse. However, if it refers to intercourse, it is not permissible to abandon it except with the wife's consent. Otherwise, the obligation is either to give her conjugal rights, or to separate from her, or to reach a reconciliation where she waives her right.