In the text of the birkas eirusin (see Kesubos 7) we say "v'asar lanu es ha'arusos," that conjugal relations are forbidden between a man and his betrothed (i.e., the ring was given but they didn't have chupah & sheva brachos yet. That stage is called both kiddushin and nesuin; I'll use those terms interchangably). Nearly all rishonim explain this to mean it's prohibited midirabnan. (See Rambam that even one who was mekadesh with biah is asur lavo aleha again before chuppa.) However, the Shita Mikubetzes (Kesubos 7b s.v. vz"l haRashba) brings the Raavad (personally, I'm not sure if this is the same Raavad as the Hasagos Haraavad, based on his comment at the beginning of hilchos Ishus) that when we say "v'asar lanu es ha'arusos," it means she is forbidden midioraissa. How can this be explained? My Rebbi Rav Ahron zt"l explained that first of all, the Raavad held like Rabbeinu Nissim Gaon in Tosfos Kiddushin 10a s.v. Kol that "ki yikach ish isha" is referring to nisuin, not eirusin. And second of all, the Raavad agrees with the Rambam (SHM Lo Sasaseh 355) that it is asur midioraissa to have conjugal relations with a woman without chuppa v'kiddushin, and basically the Raavad holds it's an issur aseh (the positive commandment not to have relations makes it prohibited, but lav haba miclal asei, asei - see Rambam SHM Shoresh 6). Thus conjugal relations before chuppa has taken place, even between a betrothed couple, is assur midioraissa according to the Raavad. Ramban, in a teshuva (284) says "v'asar lanu es haarusos" tells us it's only prohibited for a marriage without bracha. But without a marriage, you don't need a bracha. (Does this lead logically to R' Henkin's position [stay tuned for that]?)
Once we're on the topic of birchas erusin, why is the text of the bracha "mekadesh es amo yisrael al yidei chuppa v'kiddushin," if kiddushin (eirusin) takes place before the chuppa? (Note: the Mishna Lamelech says that the Rambam holds chuppa may take place before kiddushin. This is a daas yachid [less than a minority opinion].) Rav Moshe Soloveichik zt"l (son of R' Chaim) answered that this Bracha was instituted with the historical facts of marriage in mind (Hil. Ishis 1:1 & 4): before matan torah, there was only chuppa. No other type of marriage existed. After matan Torah, the kiddushin, our halachik framework of marriage was created, hence the bracha lists chuppa before kiddushin, because the institution of chuppa existed before the institution of kiddushin.