and boarding is stupid cos ur ship would need like a bizzilion men cos there would be sentry guns good covered places and stuf like that.
... Your being facetious, right?
Sure, you could include various defenses, but everything you do will make your
own crews life more difficult for daily operations, like defending the ship from enemy vessels. Above and beyond that, every thing you've mentioned could have a counter. Which is why naval vessels -- which
do face the threat of being boarded, even in this day and age -- focus on prepared crews and keeping the boarders off, rather than fiddling around with focused anti-boarding defenses like sentry guns and defensive choke points (not that they really need many additional defensive choke points, water-proofing helps with that a lot

)
If you want to call it stupid because getting people onboard the
space ship while its underway is impossible, I'll buy that, (except for the stupid part).
also games like home world where a midget frig can take over a massive cap ship is just stupid.
No, actually, its not stupid. A small force, trained specifically in hand to hand (rather than ship-to-ship) combat, with specially designed hardware
can take out a vessel whose crew isn't prepared, equipped, and trained to handle such threats. And given that such training would be taken away from the (vastly more important) training and equipment for the role of anti-ship combat, a foolish (or strapped for resources) navy might not provide it. This could be made a
lot worse for the defenders if they depend on controlling the ship by controlling specific command points -- bridge, engineering, etc -- and the boarders gain control of those before the crew can be mobilized to respond.
Again, none of this is stuff I'd think
likely to occur; only that its
possible. Fact is anyone but an idiot would be prepared for all conceivable possibilities -- which is why, to the best of my knowledge, the US Navy still practices a repel boarders drill despite the fact that I have
never heard of them using it (in recent history). Just like it maintains a thousand other drills for things that can happen... even if fires, man overboard, and other similar drills deal with issues they are a
lot more likely to see.