Well, if they're cool with doing that on a PG show in front of a decent sized live audience and anybody who watches WWP Thunderstrike, imagine what they're comfortable with when the cameras aren't rolling.
Also, I'd like to present a logical pathway which will doubtless do nothing to change your opinion. Both Justin and Lara (his blonde ex) were models. Therefore they both know that they're good looking. Knowing this, they both know that they could find a mate very easily if they wanted to. However, they were together for years. Thus they both must have been getting something from the relationship. Since they could very easily get sex from someone else should they want to, chances are that "something" included sex.
I can't prove it beyond reasonable doubt (mostly because as fas as I know, there's no Justin Gabriel sextape, but on a ballance of probabilities (what you need for a civil court case) he has had heterosexual sex.