The problem, if I understand it correctly, is not only that sex could trigger bleeding, but that the hormones (or something [very unscientific emoticon]) in semen can trigger contractions (which is why sex can be used to start labour). Obviously with Grade IV PP these contractions, even if they are not as strong as 'proper' labour contractions, may be enough to make the placenta start to come away... and then you're into a worst-case scenario type situation!
TBH, giving up sex for a few months is such a small thing to do for peace of mind and just to feel you are doing all you can to ensure that your baby (a) survives and (b) is born as close to full-term as possible, avoiding all the dangers of prematurity.
Again, sorry if that sounds like scaremongering.
Hospital admission was pretty grim, nobodysfool, but at least it was my first child. I was treated very well, too, on the ward (at the Royal Berks in Reading). Sometimes you just have to go with it... there's nothing you can do, so it's just a case of saying 'This is what I have to do for a few weeks/months in order for my baby to be safe'... and gritting your teeth! I amused myself by writing really REALLY long 'to-do' lists for dh!
Now that you're past 24 weeks have they said anything about giving you steroids for the baby's lungs, in case s/he needs to be born early? I had weekly dexamethosone (sp?) from 27-34 weeks 'just in case'. It's not very nice (side effects: I glowed red and didn't sleep for 24 hours!) but they do say that it makes a huge difference if your baby is born early. They may not want to bother with this, though, unless you have another episode of bleeding.
Also, are they keeping an eye on how your baby is growing? I lost what I thought were large clots, and it turns out that what had actually come out was a lump of placenta (sorry... gross, I know). Consequently, dd was quite small for her dates (theoretically she was 36 weeks and 4lb 12oz) but no-one realised that there was a bit of placenta missing till they got it out and inspected it after the c-section.
Lots to think about, and sorry if I am coming across as the placenta praevia doom-and-gloom-monger! I should add that my 'PP' dd is now 8 and an average size, very bright, very beautiful... no harm done whatsoever!