QV, please don't tell people that. I have seen far too many women with bleeding, cracked and damaged nipples, who have pesevered because someone like you has told them it is normal to hurt at first.
Yes, there is a lot we don't know about pain and breastfeeding. Some women do get sore, and despite everyone's best efforts, the soreness persists.
But for the majority of women, soreness equals something that needs to be fixed before it gets worse, and it is easily corrected. Yes, easily.
Experienced breastfeeding counsellors and others who know what they are supposed to be looking for can help the mother amend her positioning so it does no further damage to the nipple.
Too often, mothers who are sore get observed by someone who says 'hm, your positioning looks ok' and so they carry on. Almost (note that almost) every mother I see who has been told that, can be helped to have a virtually pain-free, effective feed straight away.
Pain is a sign that there is something wrong, not something to be borne for three, four, five or whatever weeks.
I can't see how it helps to tell mothers the agony is something they have to expect, and have to put up with.
The case you describe - the baby who is tongue tied - is especially someone who should not just put up with it. If TT is interfering with bf, the frenulum can be snipped, and pain free feeding can then take place (it's not always an instant 'cure' but it often helps a lot).
It's not 'unnatural' or 'abnormal' for bf to be difficult at first, and lots of people have a horrendous time that gets better as the weeks go by. But telling people to expect the worst...sorry, that's not good.