miss If you requested the sex to be known, they will tell you. Now the thing is making your peace with the results. Your baby was your baby, whichever sex it was - this is something that ties in with the gender disappointment MANY women feel when pregnant. It's a totally natural feeling - we tend to project what we think it means to be a 'boy' or a 'girl' onto the baby and then further project how what that would mean for our relationship with them, and how it makes us feel to 'produce' this child for your partner (and their expectations) and the extended family. But it's so important to remember that we can project all we like - bottom line is a boy or girl may be the complete opposite of what you expect; or may be exactly as you imagined it yet somehow doesn't feel the way you thought it would. Your baby was your baby and the real pain is that it is gone before you got to know who it was rather than 'what' it was. I know you feel it would be worse to hear 'male' but be prepared to feel as stunned/devastated to hear 'female' too. It's just going to be hard. But probably quite important for your 'closure'.
Wrt to std, if DP has 'history' then it's natural for your mind to jump to anything negative in either of your past's to try and pin the blame somewhere. As humans, we kinda want and need that blame (even if it then means forgiving). Afaik, there are very few stds that would cause a mc/birth defects - obviously the main one they test for at booking (syphilis) you must have been clear from. A primary infection of herpes is dangerous to the fetus during pregnancy but you would have know if you'd have caught that even if DP had kept quiet. Other, minor stds (or even a bad uti) can cause contractions/rupture of membranes a bit later but although I don't know all your details I don't think this is what happened with you, huh? Ok, so what I'm saying here is it's highly unlikely that this fear of yours will prove to be true, and whilst it's natural you're 'going there' try not to. Try not to feel that way about DP (we all have a past, some of it not so pretty!). MC has a nasty way of setting couples assunder as they struggle with their own feelings of guilt and/or blame... I don't think you deserve that on top of how you already feel so work on accepting it's normal but doesn't mean it's right.
Ditto 'something' you ate. There are a tiny number of things that can pass the placenta to harm the baby - listeria being one that jumps to mind. Now, if you'd have been poisoned by listeria, you'd have known ALL ABOUT IT! You'd have been very poorly. And, so, it wouldn't really just be a musing for apportioning blame, it would be your prime suspect and DP wouldn't have been looked at askew at all! 
I got very poorly with 'flu at the end of the first tri with my loss - extremely high temp. The consultant confirmed that it was likely to have caused the death of the baby (although as I turned down PM, we won't know for sure).
Are you completely prepared to hear 'It was bad luck/we just don't know'? and how do you actually feel (or think you feel) about that? For me, there was a strange comfort from knowing there was (probably) nothing 'wrong' with DS3, it was just bad luck I got so ill at a time when the baby was still vulnerable to v.high temps. On the other hand, I kinda wanted something to be wrong so that there was a 'proper' reason - there is just not really a 'winning' answer if you know what I mean? The baby is still gone - even with 'blame', even with a reason... Be prepared to hear what you think is something good (bad luck - nothing wrong with you!) and for it to not actually bring you the complete peace you imagine.
Dear miss I hope you can not only know that the stuff I say is logical and true, but also come to feel it. But know however much you feel you're stumbling through this time, however much you feel 'mememe', we are here and really do get it. Don't be afraid of voicing how you feel.
Ok, and now my tea is cold! Microwaved tea is totally yummy, right?