In Florida it is not "illegal" to begin another relationship upon separation., including one that results in sexual relations. However that relationship may become relevant to issues regarding child custody, visitation, or alimony to the extent of any dissipation of marital assets in pursuit of that relationship. I always advise clients who are in relationships or contemplating one to be discreet, to carefully consider the consequences of this conduct and to avoid allowing it to interfere with the best interests of the children, if any. Even in a relationship without children it is often the quickest way to inflame to other spouse and cause negotiations and subsequent litigation in ht case to be more difficult time consuming and emotional and to therefore think carefully before proceeding.
whether such a relationship is reprehensible is not a legal consideration, but to the extent that such conduct in its exercise becomes truly reprehensible , some of the above considerations may become involved, but no and affair in and of itself is not legally reprehensible.