There is a lot of research about the impact of sexual abuse where a dependence on the severity of the contact is considered. There are different aspects of severity, and an unbiased ordering would be the following:
But, if we consider the research about sex with children, we find a strange, reverse order of interest: there is de-facto no research which tries to differentiate volitional and unwanted sex. Seldom, the use of force is considered. Much more often we find information about incest/asquaintance/stranger. But the notion "severity" usually means sexual aspects of the contact - exposure/body contact/penetration, duration and so on. Fortunately, this ideological bias does not influence the results, and the meta-analysis of Rind et.al. 1998 shows the order of importance we have suggested (see here).
Incest is usually considered, and usually the seriousness of the harm, if detected, depends on incest. Examples: Wyatt & Newcomb 1990
Thus, in general the role of the sexual aspects is less important than assumed. Nonetheless, the hypothesis that the impact of unwanted sex is higher if the sexual aspects are more intense seems natural.
Mennen 1995 has found ethnical differences: Latina girls who experienced penetration abuse scored higher on all outcome measures compared with Black and White girls. IMO, explanation is easy: in Latin culture innocense of a woman has much higher value. Thus, it is culture, not sex itself, which causes this difference.
Mennen 1995 has found: "Penetration predicted higher levels of distress on depression and self-worth measures".
After controlling for the effects of dysfunctional parenting, even a sample size of 284 men was not sufficient to find significant effects (Collings 1995)