Do women in Poland change their surnames to a feminine form of their husbands' surnames?

The family name "Semelli" is perfectly valid in its country of origin! That it by coincidence sounds like the perjorative English word, has no merit in and of itself:-)

So what should say the people living in villages in Poland which have names like Zgniłe Błoto or Biały Kał?

Yo, there's a village in Austria called ****! They gotta deal with it too ( and they have for many centuries)!!!
Where d'ya think English gets the word?
))

Do Polish women generally like the system of changing their surnames to their husbands' surnames (or at least adding it to their names)? Are they proud of this tradition or do they wish they didn't have to change their surnames? I was always under the impression that as Poland develops as a country, the people will start to favor a less traditional lifestyle and start to change the system where women adapts to their husbands' surnames.

Of course, there are exceptions where the woman is proud to adapt her husband's surname...

My bf wants to get married so will I have to change my surname to kowalska?

Only if you want to. You can keep your own name if you wish.

What do u think he would expect? He speaks very little English

The rule is simple in Poland, when you are a woman marrying a guy, you take his last name. If you are a foreign woman marrying a Polish guy, it depends on your preferences. You can keep your old last name, or add his to it, or go by the Polish naming rules and change to his. The last decision again comes down to preferences, you can keep it as in the English speaking countries, for example Mary Kowalski (-ski is a male form in Polish language), or you can stick to the Polish naming rules and become Mary Kowalska (-ska is a female form in Polish language). If you plan on living in Poland, for the burecrautic reasons go with the Kowalska, if you plan on living in the West, the rest is up to you. I'm myself non-Polish, married to a Polish man, and we have decided to keep my old last name due to the fact that I would have to change all my diplomas, passports, IDs etc. However, all our children have his Polish name. I realised that it's important for him to have our children with his last name, not so much me, as the child's paternity may be relevant in relation to issues of inheritance and such.

That's not correct although you are right about rules being simple. Whenever you marry, whatever sex you are you have three options:
Stay with your name
Get a double name with hyphen in between
Your partner's last name
Simple as that.

Women usually take the husband's name however if they are professionals or like their maiden name they choose the double name option. Men take women's name usually only if their own is silly.

Very, very rarely the partners stay with different names.

Many wives or husbands retain their own names. This mainly applies to people in media/business where they are known by their own name (especially if they marry 3 or 4 or more times).