Dofi: Many of these Roshei Yeshiva hold by the p’sak that Shabbos (which is a m’doraysa) ends according to Rabbeinu Tam’s zman. Yet, a number of talmidim end at the earlier zman. Yet, the R”Y don’t “bar them” as a result. So there’s no reason to think you have any basis to assert that they’d bar them for what you say that they would.
How many Roshei Yeshivos can you name right now, in your next comment, that are opposed to 18 year old bochorim getting married? If you provide no names, as we both know you cannot legitimately provide any, we have our answer and can safely assume you to be incorrect.