Fun fact: vaginas are not "loosened" by repeated sexual intercourse. Vagina walls are not set structures that break down with use; they are very much elastic, being capable of significant expansion and contraction within a single session of sex.

The number one factor when it comes to vaginal tightness is arousal: the more aroused a woman is, the more "loose" she will be, as far as biology is concerned. So stop panicking just because you can fit into your partner easily, men – it simply means that she finds you sexy.

The reason why virgins are associated with tightness is because anxiety – often in no short supply when having sex for the first time – has the opposite effect of arousal, causing the vagina's walls to clench. This effect can be often be negated with sufficient foreplay, but because the man is often also inexperienced, it typically isn't.

TL;DR If she's tight, the main (if not only) thing that it should be indicative of is that she isn't comfortable.

http://www.sarilocker.com/advice/qa.php?id=282
https://www.psychologytoday.com/blog/all-about-sex/201109/the-rare-truth-about-tight-and-loose-women

Yes, I always carry a pen, clipboard, and ruler around in my undergarments in order to conduct research on my reproductive organs before, during, and after sex.

I now await an engineer to congratulate me on my marvellously efficient method of collecting data.