Female fat (butt and thigh) is very hard to lose while estrogen levels are high. Dieitng/losing bodyfat will naturally reduce estrogen levels as bodyfat goes down thus making female fat easier to lose, but oral contraceptives over ride this natural reduction in estrogen making it very hard to lose this fat. Some thermogenics, such as ephedrine, may increase general fat burning, but they inhibit the burning of female fat, and can even antagonize the action of drugs such as yohimbine/Yohimburn. So if you're female, ephedrine will help get your upper body very lean, but doesn't do a lot for lower body fat.
Not to sound too negative here, but oral contracepives also have a negative impact on may facets of exercise adaptation, for instance:
Oral contraceptives increase fat mass and decrease aerobic capacity:
"However, Oral Contraceptive use significantly increased body weight and fat mass, and decreased peak aerobic capacity (11%, 2.53 ± 0.21 to 2.25 ± 0.18 l/min)"
http://jap.physiology.org/cgi/content/full/93/5/1698
They slow recovery from resistance exercise:
"Force recovery began 2 days post-exercise in the non oral contraceptive group, while in the oral contraceptive group strength did not start to return to normal until 5 days post-exercise (p < 0.05). Women taking oral contraceptives had a delayed strength recovery after eccentric exercise."
http://www.ncbi.nlm.nih.gov/entrez/...ve&db=pubmed&dopt=Abstract&list_uids=12169383
They also increase triglycerides and cortisol during exercise:
"In summary the synthetic ovarian steroids found in Oral Contraceptives increase triglycerides and plasma cortisol concentrations in exercising women."
http://www.ncbi.nlm.nih.gov/entrez/...ve&db=pubmed&dopt=Abstract&list_uids=14990561
They inhibit fat-burning during exercise:
"The increase in whole body lipolytic rate during exercise noted previously with oral contraceptive use is not matched by an increase in fatty acid oxidation and results in an increase in re-esterification"
http://www.ncbi.nlm.nih.gov/entrez/...ve&db=pubmed&dopt=Abstract&list_uids=15618322
And of course dramatically reduce testosterone and other androgens is almost certainly related to all of the above.
"All steroidal serum parameters (testosterone, free testosterone, dihydroxytestosterone, androstenedione, DHEA-S, 17OHP, Alb) fell significantly during the six cycles of oral contraceptive treatment (ratio of decrease, 1.3-3), regardless of OC type. These findings show that oral contraceptive use changed the endogenous androgen environment downwards in the direction of hypoandrogenism."
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=8689882
Not to mention reduced libido but that has nothing to do with exercise...hang on, maybe it does
"Most patients using birth control pills present with decreased libido"
http://www.ncbi.nlm.nih.gov/entrez/...ve&db=pubmed&dopt=Abstract&list_uids=14740535
On the positive side, they prevent pregnancy