According to what we wrote, the "קיחה" that it says in the pasuk about marrying a woman, denotes a קנין [see the sugya at the beginning of Kiddushin and in תוספות ד"ה כתיב] which applies to Jews and not בני נח. Now we can understand Rashi in Parshas Vayera [19-14] based on the Medrash [פ"נ י"ד] that when it says about לוט 

"וידבר אל חתניו לוקחי בנותיו". 

The word חתניו, explains Rashi, refers to his two fully married daughters [נשואות] while לוקחי בנותיו refers to his daughters who were merely betrothed [מאורסות]. The Mizrachi and the Gur Aryeh tried hard to figure out where Rashi got that from. The מדרש which is Rashi's source notes that the pasuk says "לוקחי" and not "לקוחי", as a continuation of the comment that his daughters were betrothed. What is the difference between לוקחי and לקוחי?? [See the Matnos Kehuna there]. However, like we said, קיחה of אירוסין [or קידושין] denotes קנין and that doesn't exist for בני נח strictly speaking even though they tried to perform אירוסין anyway [see Bava Basra 16b that עשיו had relations with a with betrothed girl and תוספות ד"ה בא על נערה המאורסה]. The term "לקוחי" implies that they are actually legally "acquired", while לוקחי implies that they performed a מעשה קנין which was NOT effective. They were "taken" - "לוקחי", but it didn't work. So the מדרש is saying that it could not have said לקוחי because that would mean that the "taking" and קנין took effect - but that can't be true for בני נח. Instead, it was merely "לקוחי" - they were taken in אירוסין but halachically speaking it was nothing, because בני נח don't have אירוסין.

It emerges that קידושי בני נח do not involve a קנין of the husband but rather a "מצב" of אישות, a monogamous, equal relationship, agreed upon by the man and the woman. According to this, when Rashi said [Sanhedrin 57a] that if one takes a יפת תואר who is an אשת איש he transgresses the איסור of גזל, we can't say it literally means גזל because the Gentile woman is not קנויה to her husband. בני נח have no קנין in their spouses, so where is the גזל?  Even regarding a Jewish wife in whom there is a קנין, there is no monetary acquisition involved that would make גזל relevant [עי' רשב"א קידושין ו:, אבני מילואים מ"ה א', ובתשובותיו סי' י"ז, ובראשונים בשיטה מקובצת ריש כתובות ועוד ועוד], so CERTAINLY גזל is not relevant for בני נח where there is no concept of קנין at all. So Rashi means something else - see the explanation in the באר מרים פ"ט ה"ז for the explanation.

Our analysis yields the conclusion that the question of divorce for בני נח depends on the question as to whether they have a קנין in marriage - which they don't because that is unique to Jewish marriage alone. All they have is the "מציאות" of "והיו לבשר אחד", a "מצב" of matrimony without the attendant קנין that Jews have. This can then be dissolved by either party walking away. 

With this we can explain the words of the Ritva [.יבמות צ"ח] and the Nemukei Yosef [ibid] who say that when a husband and wife convert to Judasim, she becomes permitted to the public [because they are "new people"] and there is no concern that people will say that they went from a קדושה חמורה [where as a non-Jewess she was forbidden to all other men] to a קדושה קלה [as a Jewess where she is now permitted]. Otherwise [if we don't say that then], even divorce wouldn't permit her because בני נח don't have divorce. What does this mean?? The Bris Yaakov [in סימן מ"ו] writes that these Rishonim imply that בני נח don't have divorce and that is a very difficult assertion in light of the Yerushalmi and Rambam to the contrary. 

Based on the foregoing we can explain the Ritva and N"Y who say that the marriage is dissolved with the גירות, that the discussion is if a בן נח has a קנין in his wife and once it takes effect it doesn't evaporate with גירות and the new status of קטן שנולד, just like other קנינים of בני נח who convert don't evaporate. Or it is not a קנין but just a מציאות, a reality of a monogamous bond between a couple, and this מציאות only exists when they are non-Jewish and when they convert is automatically dissipates. On that query they said that we HAVE to say that it dissolves because if we don't and instead say that there is a קנין involved, then how would they ever be able to get divorced?? For בני נח are not able to divorce in such a way that unravels a קנין! From the fact that they CAN get divorced, it must be that their union is a מציאות that can be dissolved if either of them decides to walk away, as the Yerushalmi and Rambam say. 

It is also axiomatic to them that if the husband dies, she is permitted. The Pnei Yehoshua [Kiddushin 13b] says, on the contrary, there is no היתר of מיתת הבעל [because a Jewish woman is not permitted when her husband dies because there is no husband but because there is a היתר מיתה - see שערי יושר ז' י"ב]. So how do these Rishonim know that there is a היתר of מיתת הבעל? It must be that no היתר is necessary, because with his death, the מציאות of their union is automatically dissolved. If so, the same applies to divorce. Not a הפקעת קנין but a הפקעת מציאות!