The "conditional gift theory" is currently the majority viewpoint, because that is the theory expounded in the Restatement of Restitutions.An interesting relatively recent ruling on the "conditional gift" theory is Marshall v. Cassano; 2001 NY Slip Op. 40320U. This is what I would call a "conditional gift given in pari delicto," i.e. the plaintiff gave the gift in contemplation of marriage while he was still married; and the defendant knew he was married.(Under a Contracts theory, it would have been an "illegal contract," to wit, "Contracts" illegal; which of course means "contrary to public policy," not necessarily "you are going to jail.")Under the above case of what I would call, "conditional gift/in pari delicto," the fiancee' keeps the ring. In Contracts (in pari delicto), the judge would probably return the parties to where they were before the contract was formed, which I would assume means the groom gets the ring back, and the fiancee' can accept new proposals.Many jurisdictions use the objective contract theory--but some courts are reluctant, because what constitutes a "breach of the terms" of the engagement? E.g., is a rowdy bachelor party a "breach"?One last theory that would be the clear minority opinion but I believe is still in use would be the seisin/"transfer of title" theory.In this theory, which was developed in ancient Anglo-Saxon law, the bride is sort of "one step above chattel." The groom is asking for "transfer of title," from I guess her father.In a similar way that one would offer, say, a lump of dirt and a twig for land, the groom is offering the ring as a symbol.Ironically, this system would favor the bride, because she keeps the ring if the "title doesn't transfer" for whatever reason (transfer of title meaning "marriage")--similar to a land transfer not closing, and you don't have to give the lump and the twig back.
It should be noted that the bride can elicit the conditional promise by fraud, and have to give the ring back.