Being wet is a reflex. It can happen early, and even when you are nervous. Being ready for something inside is different.
Before arousal, the walls of the vagina rest against each other, folded, and the muscles around the opening are firm. As arousal builds, blood flows into the walls and they swell, the folds smooth out, the uterus lifts, and the inner part of the vagina lengthens and widens. For most women, this is what "ready" means.
If something goes in before that, it feels like pressure from outside: muscles tighten, wetness drops, and arousal falls. Going deep too early can hit the cervix before it has lifted, which hurts.
Feeling "looser" partway through sex is usually this opening happening. It is a sign of arousal, not a flaw. Position and how relaxed you are change the feeling too, so go by comfort, not by looseness alone. Bodies differ in how clearly they show it: if putting something in feels like pushing, give it more time.
See also the G-spot area and Deep pressure: when deep is right.