And a lot of wedding threads on Mumsnet, where the Bride and Groom are being obviously more grabby, the general consensus seems to be to have a wedding which you can afford.
If you want to hire an exclusive use venue, then fine, but if the only way you can afford that venue is for your guests to then have to stay at that venue (if as the OP says there are no other accommodation options within a reasonable distance), then you should have looked somewhere cheaper and probably closer to home.
Ofc, as a guest, you should pay for your own accommodation, but I would probably balk at being told that I had no choice as to where to stay, and I would feel like I was somewhat contributing to the Bride and Grooms choice of very expensive wedding venue.
But at the end of the day, it depends how much you want to go and celebrate your cousins wedding.
What happens though, If like you, a lot of the other guests are put off going, due to cost, distance etc....do the Bride and Groom then have to suck up the costs and could they afford to do so. Seems like a risk to take.
When we got married 10 years ago in a small country house hotel, which was actually just outside the city we lived in, I think it had only 20-25 rooms, and they stated they would not book anyone not in the wedding party into the hotel on that night (yet we certainly did not have to pay for the rooms ourselves and then claw back the money). A lot of our guests were from the city we live in, but my family travelled from my home city, so we paid for their hotel rooms for the wedding night, as they had already paid a lot just getting to us. The hotel had said anyone else wishing to book, had to ring and say they were of the wedding party and they got a discount too. I think on the day of the wedding they had only 4 rooms left empty, as a lot of people had decided to stay, and they left those open, in case some guests decided to book on the day, as a couple of people ended up doing so, so only 2 empty rooms. Not sure when this exclusive use business started. I loved the hotel we got married at.