Compiling the guest list for your wedding is a logistical and social nightmare. You've got to keep numbers at a reasonable level - AKA avoid bankruptcy - while accommodating both you and your fiancé's families. It's a tall order.
1. Don't feel obliged to invite partners to the whole day. Unless they're actually good friends, send them an evening invitation and be done with it. Listen, at the end of the day, do you want to look back at your wedding pictures and see a bunch of people that a) you didn't know in the first place and b) aren't going out with your friends anymore? No. I had to tell a handful of people they couldn't bring their girlfriends/boyfriends and learned this: if they're a good pal, they'll understand.
2. Don't be bullied by either side of the family. So your mother-in-law is kicking off because you can't invite all her friends? And your own mum is pressurising you to include your long-lost-cousin and her five kids? Tell them both to naff off - it's your wedding, your guest list.
Explain to both sides of the family you will try to accommodate them but can't invite everyone.
3. Do explain your decision to exclude someone if you think it may cause a rift. If you decide it's impossible to include a friend's boyfriend for the whole day, call them and be open about the tough time you're having. That way when the invitation to the wedding arrives addressed to just one person, it won't seem like a vicious snub. I found disclosing how much I was being charged per head really increased understanding.
4. Make it clear both sides of the family are compromising. So your aunt's mad your cousin's boyfriend isn't invited? Explain it's the same rules for your fiancé's side of the family.
If either side get a whiff that there might be some preferential treatment and they're not benefitting from it, all hell will break loose. Nothing kills that wedding day vibe like barely concealed resentment.
5. Remember it's your day. Therefore your guest list has to be full of people you and your fiancé want to see and love. It might mean friends and family get a bit huffy. It could result in confrontation. However, the good people in your life will understand because they want you to have the best day possible and they are your real friends. Fact.