Not ridiculous at all and perfectly achievable . It's less than two stone. Portion control, carb control, minimum sugar and no alcohol. Exercise three or so times a week. You'll easily do it provided you stick to whatever plan you choose
I think it's a bit ridiculous at what is already quite a stressful time to try and lose over 10% of your body weight especially if you "have to" to fit into your dress, which brides often do to themselves.
If you want to lose the weight and doing so/getting most of the way there by your wedding day will be good enough then there's no harm at all. Indeed it's a great idea.
But if you're going to be crying about not being able to have a glass of wine at your hen do or last minute starving to fit into your dress, then please think again.
It's definitely achievable if you stick to it religiously.
Are you getting married in 3.5 months? The only problem I see is that you will need to buy the dress before and you definitely shouldn't by a dress that is too small that you can slim in to. You don't know what your shape will be. If 3.5 months takes you to your 'buy a dress deadline' then that's perfect!
If you're planning on buying a traditional wedding dress you might need to get on and buy it now.
I went dress shopping five months before our wedding day and this ruled out most of the wedding dress designers. There were 2 or 3 that could deliver within 3 months. Still the one I ordered was late, first fitting two days before the wedding!
If you're getting it made you might also find they get booked up for the summer rush.
It's not at all unreasonable if you're committed and doing it for the right reasons. You'll just need to look at what habits you have now that need to change and decide how you want to go about losing the weight
I've tried numerous weight loss options over the years and they all work if you're committed, it's sticking to them longer term that's my shortfall. What I've found to be the best for me and my lifestyle is regular gym classes and a clear food plan for the week focussing on calorie and macro control - my fitness pal helps with that. The regular odd treat does no harm because you're in control of your overall calorie intake.
You can do it for sure. I lost 14kg for my wedding over 6 months and it's just calorie counting, exercise and weighing food for me. Fitness Pal and Fitbit work for me to track and keep motivated. Good luck x