Three points :
1. I think the reference to sexual asphyxia refers to the deliberate attempt to heighten sexual pleasure by deprivation of oxygen. The person being the throttled is the one whose pleasure is (allegedly) increased. No one is claiming or suggesting that that happened here, but it is noted in passing that it is one cause of (accidental) death by strangulation.
2. VT must have had a motive for assaulting JY. The need to silence her cries is not a credible motive because obviously she was screaming as a result of being assaulted. We cannot prove the motive and the prosecution doesn't have to, but it is reasonable to suggest what is by far the most likely motive - sex.
3. VT denies sexual motivation but admits that he wanted and tried to kiss her on the mouth after ten minutes of conversation which he took to be flirtatious. This is completely ridiculous. A girl was flirting with me, I felt the desire to kiss her on the mouth, I did so, but of course I didn't feel the slightest stirring of the old flitch and never had the slightest intention of going beyond the snogging stage. NO ! VT's admission of the kiss motive is tantamount to admitting a sexual motive the denial is self-contradictory and obviously unworthy of credence.