I have two close Polish friends, who are homosexual. They live together for many years now, in a small-ish city. (Far smaller than Warsaw or Cracow)
Form what they told me, the biggest problem with their orientation was some old ladies telling them about the "hell which await them for they are sinning".
Now, the question "should you be careful" is obviously biased - you should be careful everywhere, and every country has it's quirks - so there are certainly places where you can easily get in trouble, but that's universal - as a white man I was told to be careful in particular parts of NYC or Los Angeles.
From my own experiences in Poland, nobody ever asked me my or my friends sexual orientation and I believe that nobody really cares.
Is the "Poland, and Warsaw in particular, gay-friendly?" - Depends on the perception of friendliness, but I would say yes. There are places targeted towards such people operating for years. Whatever you read in the media, they don't get bombed or burned every week.
You have to remember, and I believe a lot of people forget about, that, in general, Eastern-Europeans are rather not outgoing and cheerful people (outside), the cultural difference if you will. So you can't expect people cheering by the fact that somebody is homosexual, but that's not a display of hostility. Indifference is, let's say, default.
To sum up - if you want troubles, you will find them, but that's universal.
Otherwise nobody will really care about your or your friends sexual preferences, unless you will try really hard to manifest it. But again, it's cultural, extensive display of any form of believes or preferences generally is frowned upon.
A final advice "on how to avoid problems, specific to Poland." - understand, that Polish (and most neighbours countries') culture is much less open and conversational in public than, let's say American one. Public display of preferences, orientations and pretty much anything else is seen as invasive and confrontational. I wouldn't say that sexual orientation is more "engaging" in such case than politics, football clubs, religion or some other topics.