Yeah, I'm with the other posters here. I'm really unsure what you want her to do? Wear a diaper for your convenience? Get some kind of (nonexistent btw) surgery? Look dude, imma learn you some for a mo cause maybe your health education was just bad I dunno.
So look, the abdominal cavity holds roughly the same volume between the sexes, depending on weight, skeleton, genetics so forth. But women, notably, have this whole extra system of organ structures they have to have in there and you know, give blood and stuff to. It's not just the uterus, there's a fair amount going on there, at least in space it takes up. The vaginal cavity + fallopian tubes and ovaries + uterus + possibly bonus endometrium tissue is an awful lot to get crammed in a space no larger than a male abdominal cavity would be.
Where do you think that space comes from?
On average (and I have at least one modern citation that isn't from entirely male researchers over 40 years ago) the male bladder holds between 300ml - 600ml more fluid than female bladders, because there's nowhere else for the bladder to expand into. This is completely without any medical conditions your girlfriend, who you expouse to love and care about the comfort of, may have and can't do shit about. The female bladder, varying depending on genetic or learned factors, just holds less damn fluid. Besides that! In average men and women the range between comfortable fullness can be as much as 1800ml in different people, so some peeps are just big bladder bitches yo.
Also, btw, peeing during/before foreplay means she's just legitimately turned on by you, you obtuse ass. The female urinary tract is pretty similar to men's in structure, including sometimes having bonus congenital prostate tissue! This doesn't mean she has sperm, she still has ovaries not testes, but her body may well produce small amounts of the seminal fluid sperm is transported in. Regardless, many women feel a sense of 'urgency' when they actually orgasm for realsies (depending on the woman, there could be several 'levels' of orgasm). What your girlfriend is doing here is TRYING NOT TO PEE ON YOU WHEN SHE CUMS.
In addition, women's hormones can have a smoothing effect on the bladder so she may have more urgency. When a woman is newly pregnant one of the first symptoms is needing to pee all the time. If our OP's GF is using a hormonal birth control, this might be a side effect.
True! And let's not forget urinary tract infections, which occur at a ratio of 8:1 in women:men in no small part due to the shorter length of the average female urinary tract. Almost 80% of women seeking treatment for a symptomatic UTI or other bladder related infection report having had sex in the last 24hrs. Specifically penetrative intercourse is considered a big risk factor. Luckily, there's lots of ways to help prevent UTIs!
Like peeing before and after sex to clear the urinary tract of any e.coli introduced by your partners penis (or toy or mouth, but seriously e.coli from the perianal region of your partner is the by far most common culprit).
Oh wait, that's inconvenient to the OP...
And her period. She probably needs to change her tampon. It's clear some of these trips involve this.
Additionally, she may be like some of us who get the period shits. I'm pretty sure she doesn't want to go into detail about that...
I started going into detail to my husband. He's super body-positive and listens to what happens. He makes fun of me sometimes for taking a while in the bathroom, but he knows why I do and I know he's kidding. Now, I come out of the bathroom after period shits making jokes about the massacre. Poor OPs girlfriend. He's so flipping judgemental, who knows if she'll ever be comfortable enough to share.