As a hetero zoo into canines, and after being around dogs most of my life (whether there was sex involved or not), plus some breeding experience and one exclusive, long-term canine lover many years ago, it seems pretty clear to me in videos - via facial expressions and body language - if a female dog is enjoying herself and/or doesn't care, or if she is not happy and/or is in discomfort/pain.
I cannot express enough how important it is for anybody who is thinking of having sex with bitches to
educate themselves on size, anatomy, heat-cycles and sexual behavior. You can't just lift up her tail and shove it in. You will most likely hurt her and/or make her afraid of the activity (or get yourself bitten). I won't get into details here since there is plenty of good information in the
How To's and Educational forum, and any search online for "canine sexuality".
I will say that those who don't care for the timing and well-being of their girl can go fuck themselves. One MUST be sure she is ready and showing the signs of being willing to try sex. It is a process - especially if she is intact but has never been previously bred - and it can be rewarding for both you and her if approached thoughtfully. The good news is, once you've gotten past the initial process (which can be both fun AND sexually frustrating) and she is okay or even happy with your cock inside her it's pretty smooth-sailing after that whenever she is in heat. She may even be willing to do it when she's not in heat if you've made it an enjoyable experience for her. It takes time and self-restraint; you just need to be patient, smart, and keep your eyes on the prize.