I am 67 years of age and I feel urge for sexual intercourse. My penis is of average size (6.5 inches length and same size of width on erection) my problem is that as soon as I enter my penis in vagina of my partner it shrinks and lost its hardness. My wife prefers me on her up, whereas when I intercourse in sitting position my penis remains hard and long but my wife does not prefer this position. I am compelled to masturbate frequently and my penis remains hard and long till end of the process. I want to know whether is there any problem with me or my wife. Till now I have sex relations with my wife only.