Penetration is hard when I'm semi-erect, so usually more foreplay, especially her going down on me, is needed otherwise intercourse isn't going to happen. If I am hard enough for penetration, but not fully hard, it's a 50/50 chance that I will either get fully hard, or sadly, lose my erection. I'm not 20 years old anymore, and sometimes life has an impact on erections. Fortunately it's not a common occurrence, and when I'm truly hard during foreplay, my wife definitely notices it and wants me deep inside her as soon as possible!